A 57 year old female patient was diagnosed as primary hyperparathyroidism after incidental finding of hypercalcemia. She was treated with radioiodine for Graves disease twenty years ago. Preoperative localization procedures with sonography and CT were unrevealing but 201 thallium scintiscan clearly demonstrated ectopic parathyroid adenoma which was confirmed by surgery. Brief review of the preoperative parathyroid localization procedure was done and the relationship between radioiodine therapy and hyperparathyroidism was discussed.