This animation describes the continued growth of the initially benign thyroid tumors causing hyperthyroidism in cats during their medical management with methimazole (Tapazole). Minimally this continued growth results in the need for progressively increasing doses of methimazole to control the patient's thyrotoxicosis. In addition, a percentage of these initially benign tumors will undergo malignant transformation to become thyroid carcinomas. Radioiodine therapy can be used to successfully treat cats with all sizes of benign and malignant thyroid tumors, but is best used on cats with small benign tumors.