Well-differentiated follicular carcinoma causing thyrotoxicosis is a rare entity. The age and sex distribution is no different from that of other patients with follicular carcinoma, with 87% older than the age of 40 and a female:male ratio of 3:1. The clinical presentation is similar to that of Graves' patients except that evidence of metastatic disease is often present (soft tissue masses, bone pain). The metastases are in the usual locations (bone, lung, mediastinum) and are often bulky. Despite the poor efficiency of iodine uptake and thyroid hormone production, the large tumor mass is capable of producing excessive hormone. Laboratory data confirm the hyperthyroid state, but the occurrence of T3 elevations with normal T4 levels is common, and T3 toxicosis may be missed if only T4 levels are measured. The role of thyroid stimulating immunoglobulins is still evolving, but such stimulators may support the growth of metastatic thyroid carcinoma and promote the development of hyperthyroidism. The treatment of these patients varied. Most had thyroidectomy followed by 131I therapy. Dosimetry allows for the administration of the largest dose of 131I with acceptable side effects. A good response to radioiodine predicted a more favorable outcome. The survival of patients with metastatic thyroid carcinoma causing hyperthyroidism does not differ from euthyroid patients with metastatic follicular disease (10-year survival, 59%).