When T 3 and T 4 reach appropriate levels in the
BLOODcirculation, the hypothalamus ceases TRH
production and the thyroid hormone cascade
tapers off.
Hyperthyroidism may result from a dysfunction
of the thyroid hormone cascade (usually a failure
of the pituitary gland to appropriately produce
TSH) or overactivity of the thyroid gland. Pituitary
ADENOMAis the most common cause of TSH-based
hyperthyroidism. Pituitary adenomas secrete TSH
that is more potent than normal TSH, eliciting a
stronger response from the thyroid gland. Hyper-
thyroidism that arises from overactivity of the thy-
roid gland may have various causes. Among them
are
•GRAVES’S DISEASE, an autoimmune disorder in
which the IMMUNE SYSTEMproduces antibodies
that attack thyroid tissue
- damage to the thyroid gland resulting from
radiation exposure (including RADIATION THERAPY
involving structures of the neck, lower face, or
upper chest) - thyroid nodules (nearly always noncancerous)
- excessive consumption of iodine, often result-
ing from medications that contain iodine (such
as the antiarrhythmia medication amiodarone) - THYROIDITIS(INFLAMMATIONof the thyroid gland)
- excessive consumption of therapeutic thyroid
hormone supplements (also called thyrotoxico-
sis factitia) - THYROID CANCER, which is rare
A dangerous weight loss practice is the
intentional consumption of thyroid
HORMONE supplement by people who
have normal thyroid function. Though
this accelerates METABOLISMto generate
weight loss, the resulting state of hyper-
thyroidism can cause serious distur-
bances of the HEART’s rhythm
(ARRHYTHMIA) and the life-threatening
condition THYROID STORM, which
requires emergency medical treatment.
Only people with diagnosed HYPOTHY-
ROIDISM (underactive thyroid) should
take thyroid hormone supplement, and
only at the DOSEthe doctor prescribes.Symptoms and Diagnostic Path
Hyperthyroidism tends to develop over weeks to
months with few indications until thyroid HOR-
MONElevels become significantly elevated, such
that the condition may be quite advanced by the
time the person becomes aware of symptoms. The
most common symptoms of hyperthyroidism are- racing PULSEand PALPITATIONS
- weight loss
- feeling hot and intolerance to environmental
heat - moist, warm SKIN
- irritability, anxiety, and insomnia (difficulty
sleeping)
GOITER (enlarged thyroid gland) is common.
People who have Graves’s disease may also have
EXOPHTHALMOS(bulging eyes, also called poptosis)
and autoimmune symptoms involving the SKINand
other body systems. The diagnostic path begins
with blood tests to measure the levels of the thyroid
hormones in the blood circulation. In hyperthy-
roidism that originates with the thyroid gland, T 3
and T 4 are usually elevated and TSH is lower than
normal. When there is a dysfunction of TSH or the
thyroid hormone cascade, T 3 and T 4 are elevated
and TSH is normal. Further diagnostic procedures
to determine the cause of the hyperthyroidism may
include ULTRASOUNDor radioiodine scan (also called
radioactive-reuptake scan) of the neck to detect
nodules or inflammation of the thyroid gland.Treatment Options and Outlook
Treatment targets reducing thyroid hormone pro-
duction. Such an approach may include antithy-
roid medications, radioactive iodine, or surgery to
remove part or all of the thyroid gland. The appro-
priate treatment depends on the circumstances of
the hyperthyroidism and the person’s overall
health status. Antithyroid medications, commonly
methimazole and propylthiouracil (PTU), work by
interfering with thyroid hormone synthesis and
with conversion of T 4 to T 3. Antithyroid medica-
tion therapy must be continuous; most people
experience a return of hyperthyroidism when
they stop taking the medications.
Radioactive iodine,^131 I, which destroys thyroid
tissue, and surgery to remove part or all of the140 The Endocrine System