[Hyperparathyroidism after radioactive iodine therapy for Graves' disease: a case report]

Herein we report a 36-year-old man with hyperparathyroidism and a past history of internal irradiation to the thyroid. Twelve years previously at age 24 years he had received 8 mCi of radioactive iodine for Graves' disease. An additional dose of 4 mCi was required 3 years later. A right lower parathyroid adenoma (28 X 23 X 20 mm, 5.7 g) was found at neck exploration. Although the association of external ionizing radiation to the head and neck and the subsequent development of hyperfunctioning parathyroid glands has been described in recent years, there are only 4 cases in the literature of parathyroid surgery for hyperparathyroidism secondary to earlier treatment with radioactive iodine for Graves' disease. In a long-term follow-up of 180 patients treated with radioactive iodine for Graves' disease, neither hypercalcemia nor hypophosphatemia was found. Whether internal radiation therapy can be a causative factor in the development of hyperparathyroidism should be elucidated in future. However, it seems reasonable to suggest that patients whose hyperthyroidism has been treated with radioactive iodine should have their serum calcium levels examined at 5-year intervals.