UAL CHARACTERISTICS and reproductive maturity.
The functions of the sex glands taper with aging,
most prominently in women to define the conclu-
sion of FERTILITY (MENOPAUSE). Though men can
remain fertile throughout their lives, testosterone
levels begin to gradually diminish in the mid-30s.
Still other endocrine tissues function only under
special circumstances, such as the PLACENTAwhich
secretes dozens of hormones that regulate preg-
nancy.
The most common endocrine disorder in the
United States is DIABETES(known clinically as dia-
betes mellitus). Approximately 13 million Ameri-
cans know they have diabetes, and health experts
believe another 5 to 6 million more have diabetes
though do not yet know, more than half of whom
are over age 60. Diabetes is a significant health
influence among adults as the leading cause of
heart disease, kidney disease, blindness, and nerve
damage (NEUROPATHY). Diabetes accounts directly
for more than 70,000 lives lost each year, making
it the sixth leading cause of death in the United
States. Some people are able to manage their dia-
betes through lifestyle factors such as diet, exer-
cise, and WEIGHT LOSS AND WEIGHT MANAGEMENT.
Others must take medication such as oral antidia-
betic medications or insulin injections. Type 1 dia-
betes is an autoimmune disorder in which the
immune system attacks the islets of Langerhans,
killing the cells that produce insulin. Type 2 dia-
betes typically develops in midlife or later, nearly
always as a consequence of INSULIN RESISTANCEaris-
ing from lifestyle factors. Health experts believe
most type 2 diabetes is preventable.
Also common is hypothyroidism (underactive
thyroid). About 5 million Americans know they
have hypothyroidism. Hypothyroidism affects
numerous body functions, including heat regula-
tion, heart rate, blood pressure, body weight, fer-
tility, energy levels, and sleep quality. As with
diabetes, health experts believe many more—per-
haps another 10 million—have the condition and
do not yet know. Hypothyroidism is more com-
mon among women and becomes more frequent
with advancing age, with some health experts
estimating as many as 20 percent of women over
age 60 have the condition.
OBESITY, in which body weight due to excessive
body fat is 20 percent or more greater than healthy
weight, is a complex confluence of endocrine,
genetic, and lifestyle factors. Much research focuses
on the role of hormones in body functions related
to APPETITE(the desire to eat) and metabolism.
Research in the 1990s identified two hormones,
leptin and ghrelin, that strongly influence appetite.
The stomach secretes ghrelin, and adipose (fat) cells
throughout the body secrete leptin. The hypothala-
mus perceives the changing levels of these hor-
mones in the bloodstream as signals of either
hunger or satiety (sense of fullness), and corre-
spondingly accelerates or decelerates digestion and
metabolism (use of energy). This research holds
intriguing implications, especially for type 2 dia-
betes. Doctors know that 90 percent of people who
have type 2 diabetes also have obesity and estimate
that a comparable percentage of people who have
obesity also have either insulin resistance or dia-
betes. Health experts estimate obesity affects 15
percent of American children and 30 percent of
American adults.Traditions in Medical History
For centuries mystery shrouded the very existence
of the endocrine glands and their functions. Until
human autopsy (cutting open the body after
death) became ethically and legally acceptable,
doctors learned of endocrine glands or their func-
tions only unintentionally. Physicians knew the
signs consistent with the diseases of endocrine
dysfunction but lacked the understanding of their
causes and thus could not treat them. Historical
records dating from ancient Mesopotamia, India,
Greece, Rome, and China, for example, document
the universal manifestation of diabetes, the first
recognized, and even today the most common,
endocrine disorder. In diabetes, the islets of
Langerhans, collections of endocrine cells in the
pancreas, stop secreting insulin, the hormone that
“unlocks” cells to allow glucose (sugar), their pri-
mary fuel, to enter. As a result, glucose accumu-
lates in the blood while cells literally starve to
death.
Antiquarian healers diagnosed diabetes using
the same concept modern doctors use (though
with vastly different methods), testing the URINE
for sugar. Because blood glucose levels rise with
inadequate insulin presence, the kidneys attempt
to restore the balance by extracting glucose from102 The Endocrine System