Also termed tetraiodothyronine, nicknamed is thyroxine. Secreted by the thyroid along with T3 (triiodothyronine...also called thyroxine), this thyroxine is mostly conjugated in the blood by TBG (thyroxine-binding globulin), whereas the more active T3 tends to float free. T4 is broken down to T3 and forms a stable feeder reserve, preventing rapid shifts in its more labile relative... t4
(T3) The substance which exerts the physiological action of thyroid hormone (see THYROID GLAND). It is formed in the body cells by the de-iodination of THYROXINE (tetraiodothyronine) which is the active principle secreted by the thyroid gland. It has also been synthesised, and is now available for the treatment of hypothyroidism (see THYROID GLAND, DISEASES OF). It is three times as potent as thyroxine.... tri-iodothyronine
(tetraiodothyronine, T4) n. the principal hormone synthesized and secreted into the bloodstream by the thyroid gland (see thyroid hormone). Most of it is converted to a more metabolically active form, *triiodothyronine, within the peripheral tissues. Both hormones act to increase the basal metabolic rate. A preparation of thyroxine (levothyroxine sodium) can be administered by mouth to treat underactivity of the thyroid gland.... thyroxine
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