Two patients had hyperthyroidism secondary to functioning metastatic follicular thyroid carcinoma. Failure to control the thyrotoxic state with antithyroid drugs in one patient prior to treatment with radioactive iodine led to exacerbation of the thyrotoxicosis with thyroid storm and death. A second patient, in whom a euthyroid state was induced before radioactive iodine therapy, had a benign posttreatment course. Radioactive iodine therapy of functioning metastatic thyroid carcinoma should be administered with caution and only after adequate preparation of the elderly patient with cardiovascular disease.