Risk Factors and Preventive Measures
The primary risk factor for papillary and anaplastic
thyroid cancers is previous radiation therapy to
the neck, lower face, or upper chest. Radiation
exposure causes about 80 percent of papillary thy-
roid cancers. Family history may establish a risk
for medullary thyroid cancer, as does having
MEN. Risk factors for anaplastic thyroid cancer are
unknown. Early diagnosis is the most effective
measure for successful treatment.
See also CANCER TREATMENT OPTIONS AND DECI-
SIONS; GOITER; SURGERY BENEFIT AND RISK ASSESSMENT;
THYROID NODULE; THYROID STORM.
thyroid gland An ENDOCRINE GLANDthat spreads
across the front of the THROATsomewhat in the
shape of a butterfly. Reddish brown in color, the
thyroid gland has two lobes that equally produce
the hormones CALCITONIN, THYROXINE(T 4 ), and TRI-
IODOTHYRONINE(T 3 ), as well as a number of precur-
sor (inactive) thyroid hormones. The cells
responsible for thyroid hormone production are
the thyroid epithelial cells, also called follicular
cells, which appear in clusters called thyroid folli-
cles. The follicular cells are the only cells in the
body that take in iodine, a mineral essential for
thyroid hormone formation. Interspersed among
the thyroid follicles are the parafollicular cells, also
called C cells, which synthesize calcitonin.
The Thyroid Hormones: Metabolic Regulation
About 90 percent of the thyroid gland’s hormone
production is T 4 , so-called because its chemical
structure contains four iodine molecules. The
other 10 percent is primarily T 3 (a structure of
three iodine molecules) along with a number of
minor hormones with unknown functions in the
body. Because the thyroid hormones are not water
soluble, they leave the thyroid gland attached to
protein carriers the LIVERproduces called thyroid-
binding globulin (TBG). The TBG transports the
thyroid hormones through the BLOOD to cells
throughout the body.
All cells have receptors for T 3 and T 4 , as these
thyroid hormones regulate cellular METABOLISM
(the exchange of energy within the cell). The thy-
roid hormones are the only peptide hormones
that can pass through the cell membrane to acti-
vate receptors within the cell cytoplasm. T 3 and T 4
appear to concentrate the enzymes that regulate
the transfer of energy within cells. T 3 is about 10
times more potent than T 4 , though about one
tenth as abundant in the blood circulation.
Researchers believe that after T 4 binds with cell
receptors it drops an iodine molecule to transform
into the more active T 3 ; however, they do not
understand the precise mechanisms by which this
takes place. T 3 that binds with cell receptors
remains T 3.
On a larger scale the thyroid hormones regulate
the body’s metabolism as well, controlling how
the body uses energy. The thyroid hormones regu-
late body temperature, lipid and carbohydrate
metabolism, HEART RATE, the force of the HEART’s
contractions, and normal growth and develop-
ment. Adequate thyroid hormone levels are also
necessary for FERTILITY(HYPOTHYROIDISMis one of
the most common causes of INFERTILITY) and for
cognitive function.
Thyroid hormones are critical for normal BRAIN
development in the unborn child as well as
throughout childhood. Rarely a child is born with-
out a thyroid gland or with a severely dysfunc-
tional thyroid gland. This establishes congenital
hypothyroidism, formerly called cretinism, a syn-
drome of pronounced growth and intellectual
deficits. The damage is permanent without imme-
diate HORMONE THERAPYto provide the body with
the necessary thyroid hormones. Undetected fetal
hypothyroidism results in permanent damage to
the brain, NERVOUS SYSTEM, and other organ sys-
tems and structures. Congenital hypothyroidism is
rare in the United States because routine newborn
and child health-care standards include regular
screening for thyroid hormone levels.Calcitonin: Bone Density and Calcium Balance
The parafollicular cells of the thyroid gland pro-
duce the peptide hormone calcitonin, which regu-
lates the balance of calcium and phosphorus in the
bones and blood. The thyroid gland releases calci-
tonin in response to elevated levels of calcium in
the blood. Calcitonin binds with receptors in the
KIDNEYS, increasing the amount of phosphorus
excreted into the URINE, and in osteoblasts (cells
within the bones that create new BONEtissue),
stimulating them to accept calcium. Calcitonin
functions in dynamic balance with PARATHYROID162 The Endocrine System