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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