Hypothyroidism is one of the most common endocrine disorders
of the dog, but is diagnosed uncommonly in other
domestic animals. In dogs, the most frequent causes of
hypothyroidism are related to impaired production and
secretion of the thyroid hormones, thyroxine (T4) and triiodothyronine
(T3), which usually result from destruction of
the thyroid gland (primary hypothyroidism). However,
hypothyroidism may also result from pituitary disorders
(secondary hypothyroidism) or hypothalamic dysfunction
(tertiary hypothyroidism). Clinical signs are variable and
may include lethargy, poor exercise tolerance, obesity, hair
loss, and skin disease.