The endocrine gland formed by two lobes laying on both lateral surfaces of the trachea is called the thyroid gland. The tissue of the gland has follicle structures. The thyroids follicles are filled with a substance called colloid, and constitute a storage for the thyroid hormones (iodothyronines). The thyroid gland together with the hypothalamus and pituitary gland forms the hypothalamic-pituitary-thyroid axis, the functioning of which is based on a feedback mechanism (Capen and Martin 2003). Thyrotropin (TSH), secreted from the pituitary gland, is the hormone which stimulates the thyroid gland to secrete iodothyronines. Tetra-iodothyronine (T4; also called thyroxine) and tri-iodothyronine (T3) are transported via the blood mainly bound to transport proteins (only about 1 % of iodothyronines is free in the plasma). The main role of the thyroid hormones is the regulation of basal metabolism (Greco and Stabenfeldt 2007).
Capen C.C., Martin S.L. The Thyroid Gland. In: Pineda M.H. and Dooley M.P. (eds.) McDonald’s Veterinary Endocrinology and Reproduction. 5 th ed. Blackwell Publishing, Ames, 2003, pp. 35-70.
Greco D.S., Stabenfeldt G.H. Endocrine Glands and Their Function. In: Cunningham J.G. and Klein B.G. (eds.) Textbook of Veterinary Physiology. 4 th ed. Saunders Elsevier, St. Louis, 2007b, pp. 428-464.