Thyroxin

Thyroxin

The hormone secreted by the thyroid gland.

Mentioned in: Goiter

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thyroxine, thyroxin

a hormone of the thyroid gland that contains iodine and is a derivative of the amino acid tyrosine. The chemical name for thyroxine is tetraiodothyronine (symbol, T₄); it is formed and stored in the thyroid follicles as thyroglobulin, the storage form. Thyroxine is released from the gland by the action of a proteolytic enzyme. T₄ is deiodinated in peripheral tissues to form tri-iodothyronine (T₃), which has a greater biological activity.

Thyroxine acts as a catalyst in the body and influences a great variety of effects, including metabolic rate (oxygen consumption); growth and development; metabolism of carbohydrates, fats, proteins, electrolytes and water; vitamin requirements; reproduction; and resistance to infection.

Thyroxine can be extracted from animals or made synthetically; it is used in the treatment of hypothyroidism and some types of goiter.

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free thyroxine

the metabolically active fraction of thyroxine; abbreviated FT₄, T_f. T₄ = T_f + TBG. The amount is very small and difficult to estimate so that the amount present in serum is not used as a more accurate indicator of thyroxine status than T₄.

thyroxine-binding globulin (TBG)

most (99.95%) of the thyroxine in plasma is bound to globulin and a small amount bound to prealbumin.

thyroxine-binding prealbumin

bound to a small fraction of circulating T₄. This is the only known function of prealbumins.

L-thyroxine

see levothyroxine.