Thyroid Stimulating Hormone (hTSH) secreted by the anterior pituitary gland is a major regulating factor of thyroid hormone synthesis and secretion. It stimulates the thyroid to synthesize and release the thyroid hormones triiodothyronine (T3) and thyroxine (T4), and induces thyroglobulin production.

Serum levels of hTSH, T3, and T4 are routinely used as indicators of thyroid function. In hyperthyroid syndromes, such as thyroid adenoma, nodular goiter, and the autoimmune disorder Graves' disease, hTSH serum levels are frequently well below normal, while levels of T3 and T4 are elevated. Most cases of hypothyroidism are due to another autoimmune disease, Hashimoto's thyroiditis, in which hTSH synthesis is increased and serum levels are high, while T3 and T4 serum levels are low.

hTSH can be dissociated to yield two dissimilar subunits, α-TSH and β-TSH. The α-subunit (approximate molecular weight of 13,500 Daltons) is virtually identical to the α-subunits of the related pituitary hormones, luteinizing hormone (hLH) and follicle stimulating hormone (hFSH), and that of chorionic gonadotropin (hCG), a placental hormone. The biological activity of hTSH is dependent on the distinct make-up of the β-subunit (approximate molecular weight of 15,000 Daltons), which differs in amino acid sequence from the β-subunits of hLH, hFSH, and hCG.

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