A
thyroid function test measures the efficiency of
thyroxine and triiodothyronine production by the thyroid gland. These two thyroid hormones, which contain iodine, regulate body metabolism. If the hormones are deficient, as in the condition called myxedema,
metabolism is slowed down. If they are in excess, as in exophthalmic goiter, metabolism is accelerated. The oldest method of measuring thyroid function is the determination of the
basal metabolic rate, or BMR. The test involves measurement of the body's
oxygen requirement during a condition of absolute rest, called the basal state. The amount of oxygen is related to the amount of thyroid
hormone present. The relationship of the BMR to thyroid hormone levels, however, is indirect and inexact. The BMR test has thus been superseded by other tests, including highly sensitive measures of serum thyroid-stimulating hormone (TSH), total and free thyroxine (TP), triiodothyronine (TO), and an index of thyroxine binding protein capacity. The thyroid uptake of radioiodine is no longer used as a primary test of thyroid function but may be useful in the diagnosis of certain conditions in which blood levels of thyroid hormone are elevated.
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