Thyroid Blood Tests (cont.)
How is hypothyroidism diagnosed?
A diagnosis of
hypothyroidism can be suspected in patients with fatigue, cold intolerance,
constipation, and dry, flaky skin. A blood test is needed to confirm the diagnosis.
When hypothyroidism is present, the blood levels of thyroid hormones can be measured directly and
are usually decreased. However, in early hypothyroidism, the level of thyroid
hormones (T3 and T4) may be normal. Therefore, the main tool for the detection
of hyperthyroidism is the measurement of the TSH, the thyroid stimulating
hormone. As mentioned earlier, TSH is secreted by the pituitary gland. If a
decrease of thyroid hormone occurs, the pituitary gland reacts by producing more
TSH and the blood TSH level increases in an attempt to encourage thyroid hormone
production. This increase in TSH can actually precede the fall in thyroid
hormones by months or years (see the section on Subclinical Hypothyroidism below). Thus, the
measurement of TSH should be elevated in cases of hypothyroidism. However, there is one exception.
If the decrease in thyroid hormone is actually due to a defect of the pituitary or hypothalamus, then
the levels of TSH are abnormally low. As noted above, this kind of thyroid disease is known as "secondary"
or "tertiary" hypothyroidism. A special test, known as the TRH test, can help distinguish if the disease
is caused by a defect in the pituitary or the hypothalamus. This test requires an injection of the TRH
hormone and is performed by an endocrinologist (hormone specialist).
How is hyperthyroidism diagnosed?
Hyperthyroidism can be suspected in patients with
tremors, excessive sweating, smooth, velvety skin, fine hair, a rapid heart rate,
and an enlarged thyroid gland. There may be puffiness around the eyes and a
characteristic stare due to the elevation of the upper eyelids. Advanced
symptoms are easily detected, but early symptoms, especially in the elderly, may
be quite inconspicuous. In all cases, a blood test is needed to confirm the
diagnosis.
The blood levels of thyroid hormones can be measured directly and are usually
elevated with this disease. However, the main tool for detection of hyperthyroidism is
measurement of the blood TSH level. As mentioned earlier, TSH is secreted by the pituitary
gland. If an excess amount of thyroid hormone is present, TSH is " down-regulated" and the
level of TSH falls in an attempt to control thyroid hormone production. Thus, the measurement
of TSH should result in low or undetectable levels in cases of hyperthyroidism. However, there
is one exception. If the excessive amount of thyroid hormone is actually due to a TSH secreting
pituitary tumor, then the levels are abnormally high. This uncommon disease is known as "secondary
hyperthyroidism."
Next: Are there other tests of the thyroid gland? »
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