Hypothyroidism in dogs should be diagnosed by integrating the history, clinical symptoms and thyroid hormone assay. Affected dogs have a slow onset of decreased activity, cold intolerance, and skin changes of thickening and mild hyperpigmentation. The hair coat becomes dry and coarse, and alopecia develops in friction areas. The non-specific clinico-pathological changes of a mild anaemia and hypercholesterolaemia do not occur consistently. The easiest and best documented thyroid hormone assay is serum thyroxine (T4). The most accurate diagnosis is made using either the thyrotropin (TSH) stimulation test or thyroid gland biopsy. The condition is controlled by thyroxine replacement therapy at a dose rate of 30--50 mcg/kg/day in 2 divided doses, for the life of the dog.