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Thyrotropin, also known as thyroid-stimulating hormone (TSH), is a hormone secreted by the pituitary gland in the brain. It acts on the thyroid gland, stimulating it to produce thyroid hormones which regulate metabolism. When thyroid hormone levels in the blood rise this has what is called a negative feedback effect on the pituitary gland. The pituitary produces less TSH and the thyroid gland's secretion of thyroid hormones falls. TSH levels in the blood are often measured to assess the function of the thyroid gland.
An area of the brain called the hypothalamus controls the release of thyrotropin from the pituitary gland. It does so by releasing thyrotropin-releasing hormone (TRH), sometimes known as thyrotropin-releasing factor (TRF) or protirelin, which travels in the blood to the pituitary. Pituitary cells called thyrotrophs are responsible for the production of thyrotropin, and they have special receptors for TRH. When TRH binds to these receptors this stimulates the thyrotrophs, causing them to release thyrotrophin into the blood stream. TSH then binds to receptors on cells in the thyroid gland, triggering increases in thyroid hormone production and secretion.
Disorders of the thyroid gland can lead to abnormally high or low levels of thyrotropin in the blood. In the condition known as hypothyroidism, the thyroid gland is underactive, causing symptoms such as tiredness, feeling cold, dry skin and weight gain. As the thyroid is not functioning properly, levels of thyroid hormones in the blood remain too low and increasing amounts of TSH are released to stimulate the gland. This results in unusually high blood TSH levels.
An underactive thyroid gland is treated with replacement thyroid hormones, which raise the thyroid hormone levels in the blood and reduce thyrotropin secretion. In many countries, newborns are tested to check for raised TSH levels. This is because untreated hypothyroidism in babies can prevent normal physical and mental development. If the condition is found early enough, it can be successfully treated with replacement hormones as in adults.
Hyperthyroidism is a disorder in which the thyroid gland is too active. Symptoms such as weight loss, sweating, anxiety and tremor may develop. Excessive amounts of thyroid hormones are released into the circulation, causing the secretion of thyrotropin to fall until it reaches an abnormally low level. An overactive thyroid may be managed using drugs which block thyroid function or treatments which destroy thyroid tissue. Following successful treatment TSH levels usually rise, unless so much thyroid tissue has been removed that patients become hypothyroid.
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