Feline hyperthyroidism is currently the most diagnosed endocrine disorder in cats. It is caused by benign adenomatous hyperplasia of the thyroid gland in 98% of the cases. The two major non-surgical treatment options, administration of antithyroid drugs and radioiodine therapy, are reviewed in this article. Before treatment is initiated, special attention should be given to renal and cardiovascular function. Antithyroid drugs contain thiourylenes, which block the synthesis of thyroid hormones. Therapy should be started at a conservative dose with monitoring of the patient for side effects. Radioiodine is taken up by the hyperactive cells of the thyroid tumor. The decay with emission of bèta-particles causes local destruction of the surrounding follicle cells. Radioiodine is considered the treatment of choice in most cats.