| Thyroid-stimulating hormone
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TSH (also known as thyrotropin ) is a 28 kDa glycoprotein synthesized by the pituitary thyrotroph. It consists of two
noncovalently linked subunits and contains about 15% carbohydrate. The α-chain is identical to that found in other pituitary glycoprotein hormones and so the specificity is determined by the β-chain and the three-dimensional configuration. The synthesis of each subunit is directed by separate messenger ribonucleic acids (mRNAs) encoded by separate genes on different chromosomes. The carbohydrate side chains are complex mixtures of unmodified, acetylated, and sulfated sugars.
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| TSH is secreted in a pattern that is both pulsatile and circadian. The plasma half-life of TSH is about 65 minutes. The normal plasma reference concentration is approximately 0.4-4.0 mU/L although logarithmic transformation is required to give it a Gaussian distribution (Fig. 37.5).
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| Figure 37.5 Distribution of plasma TSH concentrations. The reference range is 0.4-4.0 mU/L. |
| TSH acts on the thyroid gland and influences virtually every aspect of thyroid hormone biosynthesis and secretion
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| TSH acts via a specific transmembrane receptor on the target cell of the thyroid gland. Binding of TSH to the receptor activates adenylate cyclase; cAMP-dependent protein kinases control virtually every aspect of thyroid hormone biosynthesis and secretion, including iodide transport, iodothyronine formation, thyroglobulin proteolysis, and thyroxine deiodination. TSH also stimulates growth of the thyroid gland.
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| Negative feedback by thyroid hormones occurs at both hypothalamic and pituitary levels. At the pituitary level, thyroxine (T4) and tri-iodothyronine (T3) inhibit TSH secretion by decreasing both the biosynthesis and release of TSH through regulation of gene transcription and TSH glycosylation. T3, the biologically active form of the hormone, is a more potent feedback inhibitor than T4.
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