Veterinary Physiology-III

Thyrotropic hormone

THYROTROPIC HORMONE (TSH)

• The thyrotropic hormone or thyroid-stimulating hormone (TSH) is a glycoprotein with two peptide sub units, the α and β. The a sub unit is non-specific, whereas b ‑sub unit shows some species speci­ficity.
• TSH has morphological and functional effects on the thyroid.
• Functionally, the effect of TSH is to increase cell size, number of thyroid follicles and also the activity of thyroid gland which includes
• Uptake of iodide (iodine trapping)
• Production and release of thyroxine by increasing iodination and coupling reactions
• Proteolysis of thyroglobulin releasing the thyroid hormones.
• TSH enhances DNA, RNA and ribosome activities, thus favours synthesis of proteins and phospholipids in thyroid follicle cells. TSH stimulates glucose oxidation to generate NADPH needed for the peroxidase reaction.
• The functional level of the thyroid is dependent upon stimu­lation by TSH from the adenohypophysis and the thyroxine level from the thyroid gland depends upon reciprocal inhibition and stimula­tion between thyroxine and TSH.
• T₃ suppresses not only TSH release but also its synthesis by inhibiting expression of the TSH gene and decreasing the number of TSH receptors.
• TSH secretion is inhibited by dopamine, somatostatin, corti­sol and GH.
• Exposure to lower environmental temperature or chilling of the animal causes the release of TRH and TSH, thus the thyroxine formation to elevate basal metabolic rate and heat production by non‑shivering thermogenesis.