Measurement of the TSH/T4 relationship.
Hyperthyroidism is almost always caused by a hyperactive thyroid gland which is producing too
much thyroid hormone (hypothalamic causes for
hyperthyroidism are extremely rare). In the typical case, excess free thyroid hormone feeds back on the
hypothalamus and pituitary, resulting in very low TSH levels. Thus low TSH is a very good diagnostic
indicator for hyperthyroidism. The opposite happens in thyroid-related hypothyroidism: low free thyroid hormone
decreases negative feedback resulting in very
high TSH. If TSH is low in the setting of low thyroid hormone, the problem lies in the pituitary or
hypothalamus.
At present, most authorities recommend that the initial workup of thyroid status
should be done by measuring TSH or TSH in combination with assay of free T4. In the past,
TSH and free T4 assays were less reliable, so a number of other thyroid tests have
historical significance (see below).