Physiology of Finfish and Shellfish (2+1)

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.

1. It plays an important role in oxygen consumption. Gold fish treated with thiourea exhibit decreased oxygen consumption. However, definite influence of thyroid hormone on oxygen consumption could not be established by surgical thyroid-ectomy and treatment of the fish with various chemicals and hormones.
2. Thyroid influences growth of fish. Fish treated with thyroid hormone by immersion or by injection shows increase in length and weight which may be due to enhanced production of growth hormone. Surgical thyroid-ectomy or radio thyroid-ectomy in salmon and trout retarded their growth.
3. Thyroid appears to be involved in CHO metabolism of fish. When the gland is active, liver glycogen is reduced. Treatment of fish with thiourea also reduces glycogen in the liver.
4. Thyroid secretion appears to play an important role in reproduction and migration of fish. Treatment with thyroxine stimulates ovarian development, probably through the pituitary gland.
5. In brown trout and rainbow trout T₃ ( triiodothyronine because it contains 3 iodine atoms) is essential for adaptation to seawater.
6. Thyroxine also promotes maturation in fishes. Scale and bone formation in fishes is also controlled by thyroxine. Both T₄ and T₃ have similar effects on target cells. In most target tissues, T₄ is converted to T₃.