Triiodothyronine is measured as part of a thyroid function
evaluation. Triiodothyronine may be measured in cases in which there is some doubt about
whether the patient has hyperthyroidism or hypothyroidism after measuring T4 and RT3U. For example, in some cases, symptoms of hyperthyroidism are
apparent but T4 is normal and Triiodothyronine is elevated.
TSH (thyroid-stimulating hormone) is a peptide hormone secreted
by the pituitary gland. It stimulates the synthesis and secretion of T4 (thyroxine) and
Triiodothyronine from the thyroid gland. TSH is, itself, stimulated by TRH
(thyrotropin-releasing hormone), which is released by the hypothalamus. Triiodothyronine
and T4 feedback inhibit the release of both TSH and TRH in normal people. Most of the
thyroid hormone secreted by the thyroid gland is in the form of T4, but Triiodothyronine
is probably the active hormone; that is, T4 is converted to Triiodothyronine by target
tissues.
T4 is the major hormone controlling the basal metabolic rate. The exact mechanisms are not
completely known, but it is known that T4 increases the concentrations of numerous enzymes
involved in the production of energy in all nucleated cells of the body.
Most of the T4 in the blood is bound to proteins; this acts as a reservoir of available
thyroxine since only free T4 is active in cells. Only 0.03% of the T4 in plasma is free
(soluble and active) and 0.3% of the Triiodothyronine is free. Most of the
Triiodothyronine and T4 are transported by TBG (thyroxine binding globulin), but smaller
amounts are found on prealbumin and albumin. The available concentrations of
Triiodothyronine and T4 are affected by the level of TBG, which is measured by the R3TU
test. T4 appears to be converted to Triiodothyronine within cells, before it enters the
nucleus and interacts direction with DNA, eventually resulting in the production of
various proteins by the cell.