The thyroid hormones, primarily THYROXINE(T 4 )
and TRIIODOTHYRONINE(T 3 ), regulate METABOLISMin
every cell. Deficiencies of these hormones cause
numerous symptoms that reflect slowed metabo-
lism. The consequences can be particularly signifi-
cant when hypothyroidism occurs in children,
interfering with physical growth and intellectual
development. Congenital hypothyroidism (for-
merly called cretinism) can cause permanent
impairments. Fortunately congenital hypothy-
roidism is rare in the United States because new-
born and well-child health-care screenings test for
hypothyroidism.
The follicular cells of the thyroid gland synthe-
size (manufactures) thyroid hormones from the
amino acid tyrosine and the mineral iodine, both
of which it acquires from dietary sources. A short-
age of either in the diet, though very uncommon
in the United States, can impair thyroid HORMONE
synthesis. The hypothalamus initiates the hor-
monal cascade that results in thyroid hormone
production, secreting THYROID-RELEASING HORMONE
(TRH) when T 3 and T 4 levels in the BLOODcircula-
tion drop. TRH stimulates the anterior lobe of the
PITUITARY GLAND to produce THYROID-STIMULATING
HORMONE(TSH), which in turn stimulates the thy-
roid gland’s follicular cells. Rising T 3 and T 4 levels
in the blood then reverse the hormonal cascade.
Sometimes the cause of hypothyroidism is
clear. Hypothyroidism is certain in anyone who
has had a total thyroidectomy (removal of the
thyroid gland, such as to treat THYROID CANCERor
GRAVES’S DISEASE) and likely in a person who has
had a partial thyroidectomy. Treatment for HYPER-
THYROIDISM(overactive thyroid gland) often results
in eventual hypothyroidism as follicular cells
within the thyroid gland continue to die after
treatment ends. The most common identifiable
cause is THYROIDITIS, an INFLAMMATIONof the thyroid
gland that destroys follicular cells and that may be
an autoimmune process. Most often, however,
hypothyroidism is idiopathic—the cause remains
unknown.
Symptoms and Diagnostic Path
The symptoms of hypothyroid appear gradually
and are often nonspecific. They typically include
- chronic tiredness
- weight gain or inability to lose weight
- DEPRESSIONand irritability
- coarsening HAIRand hair loss
- dry, flaky SKIN
- loss of eyebrow hair
- intolerance to cold or feeling cold regardless of
the environmental temperature - irregular MENSTRUATIONand INFERTILITY
Some people also have a GOITER, a painless
swelling of the thyroid gland that may be visible
when looking in the mirror or that the doctor can
feel. The diagnostic path begins with blood tests to
measure the levels of thyroid hormones in the
blood circulation. A low level of T 3 and T 4 coupled
with elevated TSH indicates the thyroid gland is
not responding to the pituitary gland’s hormonal
signals and provides a conclusive diagnosis of
hypothyroidism. Some people have borderline
blood test results though have hypothyroidism
nonetheless. When blood thyroid levels are mar-
ginal, the doctor may recommend a trial of treat-
ment to see if symptoms improve.Treatment Options and Outlook
Treatment is HORMONE THERAPYwith thyroid hor-
mone supplement to deliver adequate levels of
thyroid hormones. The most common form of
thyroid hormone supplement is a synthetic phar-
maceutical preparation of T 4 (levothyroxine). This
provides adequate thyroid hormones for most
people because the cells in the body convert T 4 to
T 3 when it binds with them. There are several
levothyroxine products available. Endocrinologists
recommend staying with the same product consis-
tently, as the formulations of each product are
somewhat different. Some doctors also prescribe
T 3 hormone supplement, which is faster-acting, in
combination with a T 4 hormone supplement, as
an approach that attempts to more precisely repli-
cate the body’s thyroid hormone synthesis. The
doctor also may prescribe a short course of T 3 to
rapidly bring thyroid hormone levels up when
hypothyroidism is severe, then taper off and
resume T 4 supplement.
Most people experience some improvement of
symptoms within two weeks of starting hormone
therapy. However, it may take six months to a146 The Endocrine System