Thyrotropin

thyrotropin (thī'rätrō`pĭn) or thyroid-stimulating hormone (TSH), hormone released by the anterior pituitary gland that stimulates the thyroid gland to release thyroxine. The release of thyrotropin is triggered by the action of thyrotropin-releasing factor (TRF), a substance found in the hypothalamus of the brain. TRF, once released from the hypothalamus, travels in the bloodstream to the anterior pituitary, where it causes the release of thyrotropin. This latter substance, a glycoprotein (see protein), is carried to the thyroid gland by the blood, where it stimulates the uptake of iodine, the conversion of diiodotyrosine to thyroxine, and the secretion of thyroid hormones into the bloodstream. Thyroxine inhibits the further release of thyrotropin by interfering with the action of TRF; thus the levels of thyroid hormones are regulated. If not enough iodine is available in the diet, then not enough thyroxine will be made to shut off the release of thyrotropin. Prolonged stimulation of the thyroid by thyroid-stimulating hormone results in an abnormal enlargement of the gland, known as goiter, a condition which has been largely eradicated by the widespread usage of iodized salt.

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thyrotropin [thī′rä·trə·pən]

(biochemistry)

A thyroid-stimulating hormone produced by the adenohypophysis.

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Thyrotropin 

(thyrotropic hormone, thyroid-stimulating hormone, TSH), a hormone produced by the anterior lobe of the hypophysis in vertebrate animals and man that controls the development and functions of the thyroid gland. Thyrotropin is a glycoprotein with a molecular weight of 28,000–30,000. It stimulates the splitting of the protein thyroglobulin in the thryoid follicles and the release of the active thyroid hormones, thyroxine and triiodothyronine, into the blood. It also promotes the enlargement of follicular cells, the absorption of iodine, and the synthesis of thyroxine.

The mechanism of action of thyrotropin, like that of several other hormones, is related to the hormone’s capacity to activate the synthesis of cyclic adenylic acid (cAMP), which stimulates the splitting of thyroglobulin. The synthesis and secretion of thyrotropin are controlled by the central nervous system and, primarily, by the hypothalamus, which produces the special thyrotropin releasing factor. When the concentration of thyroid hormones in the blood increases, the hormones inhibit by negative feedback the secretion of thyrotropin by acting both on the hypothalamic regulatory centers and directly on the hypophysis, causing the secretion of thyroxine and triiodothyronine to diminish. Epinephrine and the corticosteroids also suppress the secretion of thyrotropin, which accounts for the decrease in thyroid activity in various stress reactions (except cold stress). (See alsoADAPTATION SYNDROME and NEUROSECRETION.)

I. V. KRIUKOVA