The effects of age, concurrent illness, and administered medications complicate diagnosis of thyroid dysfunction in geriatric patients. Interpretation of thyroid hormone testing should take these factors into account. The most common thyroid disorder in dogs is acquired hypothyroidism. Therapeutic monitoring should be utilized for monitoring treatment of canine hypothyroidism. The most common thyroid disorder in cats is benign hyperthyroidism. Diagnosis is most often complicated by the presence of concurrent illness. Treatment should be individualized based on individual case characteristics and presence of concurrent illness. Some older cats have a palpable goiter months to years before development of clinical signs of hyperthyroidism.