thyroxine (redirected from free thyroxine)

thyroxine (thīrŏk`sēn), substance secreted by the thyroid gland. The hormone thyroxine forms by combining the amino acid tyrosine with iodine. Complexed to a protein, it is stored in the follicle stems between thyroid cells. Thyroxine enters into the bloodstream complexed to another protein, plasma globulin. Thyroxine increases the number and activity of mitochondria in cells by binding to the cells' DNA, increasing the basal metabolic rate. Administration of thyroid hormones, such as thyroxine, causes an increase in the rate of carbohydrate metabolism and a rise in the rate of protein synthesis and breakdown. The hormone, which excites the nervous system and leads to increased activity of the endocrine system, remains active in the body for more than a month. Thyroxine activity is controlled by thyrotropin, a substance released from the pituitary gland. Conversely, thyroxine regulates the effect of thyrotropin by feedback inhibition, i.e., high levels of thyroxine depress the rate of thyrotropin secretion. Synthetically prepared thyroxine is used clinically in the treatment of thyroid gland deficiency diseases in adults and in the treatment of cretinism in children.

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thyroxine

 or l-tetraiodothyronine or T₄

One of the two major hormones (along with the closely related l-triiodothyronine, or T₃) secreted by the thyroid gland. Its principal function is to stimulate oxygen consumption and thus metabolism in all cells and tissues in the body. Thyroxine is formed by the addition of iodine to the amino acid tyrosine while the latter is in a protein-bound form. Thyroxine secretion is excessive in hyperthyroidism and deficient in hypothyroidism.

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thyroxine [thī′räk‚sēn]

(biochemistry)

C₁₅H₁₁I₄NO₄The active physiologic principle of the thyroid gland; used in the form of the sodium salt for replacement therapy in states of hypothyroidism or absent thyroid function.

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Thyroxine

A hormone secreted by the thyroid gland. Thyroxine (structure 1 ) is quite similar chemically and in biological activity to triiodothyronine ( 2 ).

(1)

Both are derivatives of the amino acid tyrosine and are unique in being the only iodine-containing compounds of importance in the economy of all higher forms of animal life. The thyroid gland avidly accumulates the small amount of iodine in the diet. This iodine is oxidized to iodide ion in the gland and then reacts with tryosine to form mono- and diiodotyrosine. These latter are then coupled to form either thyroxine or triiodothyronine. See Thyroid gland

The maintenance of a normal level of thyroxine is critically important for normal growth and development as well as for proper bodily function in the adult. Its absence leads to delayed or arrested development. It is one of the few hormones with general effects upon all tissues. Its lack leads to a decrease in the general metabolism of all cells, most characteristically measured as a decrease in nucleic acid and protein synthesis, and a slowing down of all major metabolic processes.