Encyclopedia of Health and Medicine

140 The Endocrine System
When T 3 and T 4 reach appropriate levels in the
BLOOD circulation, 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
ADENOMA is 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. Hyperthyroidism
that arises from overactivity of the thyroid
gland may have various causes. Among them
are
• GRAVES’S DISEASE, an autoimmune disorder in
which the IMMUNE SYSTEM produces 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 resulting
from medications that contain iodine (such
as the antiarrhythmia medication amiodarone)
• THYROIDITIS (INFLAMMATION of the thyroid gland)
• excessive consumption of therapeutic thyroid
hormone supplements (also called thyrotoxicosis
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 METABOLISM to generate
weight loss, the resulting state of hyperthyroidism
can cause serious disturbances
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 DOSE the doctor prescribes.
Symptoms and Diagnostic Path
Hyperthyroidism tends to develop over weeks to
months with few indications until thyroid HOR-
MONE levels 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 PULSE and 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 SKIN and
other body systems. The diagnostic path begins
with blood tests to measure the levels of the thyroid
hormones in the blood circulation. In hyperthyroidism
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 ULTRASOUND or 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 production.
Such an approach may include antithyroid
medications, radioactive iodine, or surgery to
remove part or all of the thyroid gland. The appropriate
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 medication
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 the