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5.2.7. Functions
Thyroid follicles synthesize thyroid hormone after extracting iodine from the blood and combining it with tyrosine (an amino acid). The hormone in fish contains mono-iodotyrosine (MIT), diiodotyrosine (DIT), triiodothyronine (T3) and thyroxine (T4). The secretion of hormone from the thyroid is controlled by thyrotrophic stimulating hormone (TSH) of the pituitary gland and performs important functions in the metabolism of the fish. (The synthesis of thyroid hormones is tightly controlled. When the amount of thyroid hormone in your blood drops, the pituitary gland secretes a hormone called thyroid-stimulating hormone (TSH). As its name suggests, TSH then stimulates the thyroid gland to increase its uptake of iodine from the blood, so that, more thyroxine (T4) can be synthesized. When necessary, thyroxine is then converted to the metabolically active triiodothyronine (T3), a process that involves removing one iodine atom from T4). Surgical thyroid-ectomy is not possible in teleosts as thyroid is diffused and follicles are scattered in many organs. However, functions of this gland have extensively studied by means of chemical thyroid-ectomy (using thiourea) and radio thyroid-ectomy using I131. Inspite of the efforts of several investigators, the functions of thyroid hormone have not been fully understood. (Thiourea is an organosulfur compound of with the formula S C ( N H 2 ). It is structurally similar to urea, except that the oxygen atom is replaced by a sulfur atom, but the properties of urea and thiourea differ significantly). Experiments suggest that thyroid gland is involved in several physiological activities of the fish such as growth, osmoregulation, carbohydrate metabolism, reproduction and migration.
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