ABSTRACTHyperthyroidism was diagnosed in 24 aged cats and was characterized by palpable enlargement of the thyroid gland, high circulating levels of thyroid hormones, increased thyroidal uptake of123I, and abnormal uptake and distribution of activity on thyroid scintiscans. Unilateral or bilateral thyroidectomy was performed in all cats and resulted in remission of signs in all but one case. The histological diagnosis was adenomatous hyperplasia in 23 cats and adenocarcinoma in one. Serum thyroid hormone concentrations decreased to normal or subnormal levels within 24 hours after complete removal of functioning thyroid nodules. Hypothyroidism occurred postoperatively in 16 of the 24 cats. Presumed hypoparathyroidism and hypocalcaemia occurred in 6 cases.