dysfunctional in the unborn child, results in per-
manent damage to growth, development, and
intellect. HYPERTHYROIDISM, in which the thyroid
gland secretes excessive thyroid hormones, can
cause serious and permanent damage to the HEART
and to the eyes (GRAVES’S OPHTHALMOPATHY).
Calcium balance: the parathyroid glands Aris-
ing from the surface of the thyroid gland where it
wraps around the TRACHEA are the four tiny
PARATHYROID GLANDS, arranged in pairs on each of
the thyroid gland’s two lobes. Though each
parathyroid gland alone is barely the size of a
grain of rice, the parathyroid glands collectively
keep the heart beating, the muscles moving, and
the bones solid. The parathyroid glands produce
PARATHYROID HORMONE, also called parathormone,
which regulates the balance between calcium and
phosphate. This equilibrium is essential for the
conduction of nerve impulses in the heart and the
skeletal muscles, proper growth of the bones and
TEETHin childhood, and BONE DENSITYand STRENGTH
in adulthood. The relationship between the thy-
roid gland and the parathyroid glands is function-
ally as well as physically intimate. The release of
parathyroid hormone causes calcium levels in the
blood to rise, counterbalancing the actions of cal-
citonin.
Glucose balance: the islets of Langerhans Dis-
tributed throughout the exocrine cells that make
up the pancreas are about a million clusters of
endocrine cells ranging in size from a few dozen to
a few hundred cells. These clusters are the islets of
Langerhans, and their three distinct cell types
secrete the hormones GLUCAGON, INSULIN, and
SOMATOSTATIN. These hormones regulate the body’s
balance and use of GLUCOSE, the primary source of
fuel for many functions of METABOLISM. The islet
cells also secrete a number of other hormones
whose functions remain less clearly understood.
Reproduction: the gonads (sex glands) The
gonads (sex glands)—the OVARIESin women and
the testes in men—produce the hormones respon-
sible for sexual maturity and reproductive capabil-
ity. The gonads become active at PUBERTY, when
the hypothalamus signals the pituitary gland to
begin producing FOLLICLE-STIMULATING HORMONE
(FSH) and LUTEINIZING HORMONE (LH). These hor-
mones in turn stimulate the gonads to produce
the sex hormones. Men and women alike have all
of these hormones in their bodies; men have a
predominance of testosterone and women have a
predominance of estrogen and progesterone.
In women the ovaries produce estrogen, prog-
esterone, and a small amount of testosterone. In
men the testes produce testosterone, INHIBIN
(which regulates SPERMproduction), and a small
amount of estrogen along with a number of other
minor hormones that have narrowly specialized
functions. In men and women alike, the adrenal
cortex produces small amounts of estrogen, pro-
gesterone, and testosterone. Estrogen is important
for lipid metabolism and storage, testosterone is
important for building and maintaining MUSCLE
mass, and progesterone is an important precursor
hormone for the synthesis of other steroid (lipid-
based) hormones.
Building the immune system: the thymus In
the 1940s doctors believed there was a connection
between an enlarged THYMUSand sudden, unex-
plainable death in infants. They termed this condi-
tion status thymicolymphaticus and treated it with
RADIATION THERAPYto destroy the thymus. There
was little evidence to support this connection,
however, and doctors began to notice that chil-
dren treated with irradiation were unusually
susceptible to infection. By the 1960s, as under-
standing began to grow about the functions of the
immune system and researchers began to recog-
nize the thymus had a role in immune function,
and doctors abandoned both the concept of status
thymicolymphaticus and its treatment.
The thymus secretes the hormone THYMOSIN,
which helps the immune system’s T-cell lympho-
cytes reach maturity and stimulates blood stem
cell production in the BONE MARROW. Researchers
speculate the thymus also produces several as yet
unidentified hormones that influence immune
function. Doctors now know, too, that the normal
release of GROWTH HORMONE(GH) during the middle
years of childhood stimulates an increase in the
size of the thymus, explaining the reason for its
enlargement in young children. During this period
of development the thymus becomes particularly
active in maturing and releasing into the body the
T-cells that will form the foundation of immune
function for the remainder of life.
Circadian cycles: the pineal gland The PINEAL
GLAND, a small pinecone-shaped structure buried100 The Endocrine System