A 70-yr-old woman presented with hyperthyroidism and metastatic follicular carcinoma of the thyroid. The blood level of thyroid stimulating immunoglobulin (TSIg) was elevated. A total thyroidectomy was performed. One month later she remained hyperthyroid. Three weeks after therapy with 218 mCi of I-131 sodium iodide, the patient was euthyroid. Six months after the initial radioiodide therapy, she was again hyperthyroid and was given a second oral treatment dose of I-131 (220 mCi). Five months later, the patient had again become euthyroid. It is likely that initially the woman's metastases were producing sufficient hormone to render her hyperthyroid. After thyroidectomy and two large doses of radioiodide, she has remained euthyroid without having to take exogenous hormone. The blood level of TSIg had become undetectable. Based on this finding, we offer a tentative classification of the causes of hyperthyroidism in patients with thyroid carcinoma.