This chapter deals with hypothyroidism in horses, ferrets, rabbits, rodents, pet birds, and pet reptiles. Under category, pathogenesis, classical signs, diagnosis, and treatment are discussed. The ionized form is the biologically active form therefore it is derangements to ionized calcium levels rather than the total plasma calcium that lead to clinical signs of hypocalcemia. Hypocalcemia may develop during late gestation, the periparturient period or lactation in breeding animals as a result of increased calcium demand. The measurement of serum ionized calcium, rather than serum total calcium, provides a more precise estimate of an individual's calcium status especially in the diseased avian patient. Ensure calcium intake is sufficient, especially in high risk individuals, Providing the appropriate husbandry for the species concerned is of paramount importance.