Thyroid Function Test
Thyroid function test is done to screen and
diagnose thyroid disorders; to monitor treatment of
hypothyroidism.
The major thyroid hormone secreted by the thyroid gland
is thyroxine, also called T4
because it contains four iodine atoms. To exert its
effects, T4 is converted to triiodothyronine (T3) by
the removal of an iodine atom. This occurs mainly in
the liver and in certain tissues where T3 acts, such
as in the brain. The amount of T4 produced by the thyroid
gland is controlled by another hormone, which is made
in the pituitary gland located at the base of the brain,
called thyroid stimulating hormone (abbreviated TSH).
The amount of TSH that the pituitary sends into the
blood stream depends on the amount of T4 that the pituitary
sees. If the pituitary sees very little T4, then it
produces more TSH to tell the thyroid gland to produce
more T4. Once the T4 in the blood stream goes above
a certain level, the pituitary’s production of
TSH is shut off.
Blood tests to measure TSH, T4 and T3 are readily available
and widely used.
• TSH Tests
The best way to initially test thyroid function is
to measure the TSH level in a blood sample. A high TSH
level indicates that the thyroid gland is failing because
of a problem that is directly affecting the thyroid
(primary hypothyroidism). The opposite situation, in
which the TSH level is low, usually indicates that the
person has an overactive thyroid that is producing too
much thyroid hormone (hyperthyroidism). Occasionally,
a low TSH may result from an abnormality in the pituitary
gland, which prevents it from making enough TSH to stimulate
the thyroid (secondary hypothyroidism). In most healthy
individuals, a normal TSH value means that the thyroid
is functioning normally.
• T4 Tests
T4 circulates in the blood in two forms: 1) T4 bound
to proteins that prevent the T4 from entering the various
tissues that need thyroid hormone and 2) free T4, which
does enter the various target tissues to exert its effects.
The free T4 fraction is the most important to determine
how the thyroid is functioning, and tests to measure
this are called the Free T4 (FT4) and the Free T4 Index
(FT4I or FTI). Individuals who have hyperthyroidism
will have an elevated FT4 or FTI, whereas patients with
hypothyroidism will have a low level of FT4 or FTI.
Combining the TSH test with the FT4 or FTI accurately
determines how the thyroid gland is functioning. The
finding of an elevated TSH and low FT4 or FTI indicates
primary hypothyroidism due to disease in the thyroid
gland. A low TSH and low FT4 or FTI indicates hypothyroidism
due to a problem involving the pituitary gland. A low
TSH with an elevated FT4 or FTI is found in individuals
who have hyperthyroidism.
• T3 Tests
T3 tests are often useful to diagnosis hyperthyroidism
or to determine the severity of the hyperthyroidism.
Patients who are hyperthyroid will have an elevated
T3 level. In some individuals with a low TSH, only the
T3 is elevated and the FT4 or FTI is normal. T3 testing
rarely is helpful in the hypothyroid patient, since
it is the last test to become abnormal. Patients can
be severely hypothyroid with a high TSH and low FT4
or FTI, but have a normal T3.
Management
Proper maternal thyroid function during pregnancy is
important for the health of both the mother and developing
child. This is especially true during the first trimester,
when the fetus is completely dependent on the mother
for thyroid hormone. However, thyroid disorders are
common in women of child-bearing age. Worldwide, maternal
iodine deficiency is a common cause of thyroid dysfunction
that can have serious consequences for the mother and
child, and although progress has been made to ensure
proper maternal iodine nutrition, there are many areas
of the world (including developed countries) where pregnant
women do not have an adequate intake of iodine. Overt
hypo- and hyperthyroidism have long been known to have
a significant impact on maternal/fetal health, and in
recent years, there has been a growing awareness that
even mild or “subclinical” forms of thyroid
disease (e.g., elevated thyroid stimulating hormone
(TSH) with normal free thyroxine) are associated with
adverse outcomes for both the mother and child. Elevated
serum TSH has been associated with increased risk of
pre-term birth, placental abruption, fetal death, and
impaired neuro-psychological development in the child.
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