Primary thyroid malignancies typically do not interfere with thyroid function. A patient is reported with anaplastic thyroid carcinoma presenting with hyperthyroidism; this is believed to be the first such case described. Prior to the onset of the thyroidal process, the patient was clinically euthyroid. It is concluded that the mechanism of thyrotoxicosis is analogous to that of subacute thyroiditis: rapid tissue necrosis with resultant release of thyroid hormone.