There are four small glands (diameter between 1 and 5 mm) adherent to the thyroid gland in the dog and cat. These glands consist of chief cells (cells which produce and secrete parathyroid hormone) and oxyphil cells (without a role in the synthesis of parathyroid hormone) which together form parathyroid glands (Capen and Rosol 2003). Produced by chief cells, parathyroid hormone is one of the hormones which play a role in the control of calcium and phosphorus homeostasis (Greco and Stabenfeldt 2007). The main role of parathyroid hormone is the increase of calcium concentration and decrease of phosphorus concentration in the blood (Capen and Rosol 2003).
Calcitonin and cholecalciferol (vitamin D 3 ) are the other hormones involved in the regulation of the calcium and phosphorus balance. Calcitonin is a polypeptide hormone produced by C-cells of the thyroid gland. The main biological effect of calcitonin action is a decreasing level of calcium and phosphorus in the blood (Capen and Rosol 2003, Greco and Stabenfeldt 2007).
Vitamin D is delivered with food (vitamin D 2 called ergocalciferol) and synthesized in the epidermis (vitamin D 3 called cholecalciferol). In dogs and cats, however, cholecalciferol synthesis is insufficient, and vitamin D must be delivered with food in these species (Tryfonidou et al. 2010). The active form of cholecalciferol, called calcitriol or 1,25-dihydroxycholecalciferol (1,25(OH) 2 D 3 ) is formed by the enzymatic reactions in the liver and further in the kidney. The main role of this biologically active vitamin D metabolite is to increase absorption of calcium in the intestine and decrease of its renal excretion (Capen and Rosol 2003; Greco and Stabenfeldt 2007).
Capen C.C., Rosol T.J. The Calcium Regulating Hormones: Parathyroid Hormone, Calcitonin, and Cholecalciferol. In: Pineda M.H. and Dooley M.P. (eds.) McDonald’s Veterinary Endocrinology and Reproduction. 5 th ed. Blackwell Publishing, Ames, 2003, pp. 71-140.
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, 2007, pp. 428-464.
Tryfonidou M.A., Hazewinkel H.A.W., Kooistra H.S. Calciotropic Hormones. In: Rijnberk A. and Kooistra H.S. (eds.) Clinical Endocrinology of Dogs and Cats. An Illustrated Text. 2 nd ed. Schlütersche Verlagsgesellschaft, Hannover, 2010, pp. 253-289.