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TSH - Thyroid-stimulating hormone

Analyte: TSH

Specimen Type: For information on this assay, please contact PBI.

Biological or Clinical Significance:

Thyroid-stimulating hormone (TSH, thyrotropin) is a glycoprotein with a molecular weight of approx. 30,000 daltons and consisting of two subunits non-covalently bound. The beta-subunit carries the TSH-specific immunological and biological information, whereas the alpha-chain carries species-specific information and has an identical amino acid sequence to the alpha-chains of LH, FSH and hCG.

TSH is formed in specific-basophil cells of the anterior pituitary and is subject to a circadian secretion sequence. TSH has a stimulating action in all stages of thyroid hormone formation and secretion; it also has a proliferative effect. The determination of TSH serves as the initial test in thyroid diagnostics. Even very slight changes in the concentrations of the free thyroid hormones bring about much greater opposite changes in the TSH level. Accordingly, TSH is a very sensitive and specific parameter for assessing thyroid function and is particularly suitable for early detection or exclusion of disorders in the central regulating circuit between the hypothalamus, pituitary and thyroid.

The most important controller of TSH secretion is thyroid-releasing hormone. Thyroid-releasing hormone is secreted by hypothalamic neurons into hypothalamic-hypophyseal portal blood, finds its receptors on thyrotrophs in the anterior pituitary and stimulates secretion of TSH. Thyroid-releasing hormone is only three amino acids long. Its basic sequence is glutamic acid-histidine-proline, although both ends of the peptide are modified. Secretion of thyroid-releasing hormone, and hence, TSH, is inhibited by high blood levels of thyroid hormones in a classical negative feedback loop.

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