Three patients with hypothyroidism of 15-48 months\u27 duration developed Graves\u27 disease while on thyroid hormone replacement therapy. In one patient, the hyperthyrold phase was preceded by ophthalmopathy and nonsuppresslble 24-hour radioactive iodine uptake (RAIU). Eventually, all had hyperthyrold findings with elevated thyroid hormone levels, high 24-hour RAIU, and a diffuse uniform uptake of RAI by the enlarged thyroid. All subjects had negative antithyroglobulin and antimicrosomal antibody titers. Two had demonstrable long-acting thyroid stimulator (LATS) in their sera. All three were followed for 5-36 months while on propylthiouracil therapy prior to surgery or RAI administration. Both LATS-positive patients underwent bilateral subtotal thyroidectomy. Classical changes of Graves\u27 disease were found on histopathological examination of resected tissues
