Some Aspects of Iodine Metabolism in Radioiodine-Treated Graves' Disease

Fifteen cases of Graves' disease were studied before and after euthyroidism was achieved with radio iodine treatment. In spite of the euthyroid status, the thyroidal 131 I-uptake and PB 131 1 remained high. Measurements of the plasma inorganic iodine, the urinary iodine and the absolute iodine uptake by the thyroid showed that the increased 131 I uptake was not due to iodine deficiency, but mainly to faulty utilization of the iodine trapped by the thyroid, probably because of an organification defect. The intrathyroidal exchangeable iodine pool was decreased in the treated patients, and this could adequately explain the high PB 131 I values in these cases. These results confirm the unreliability of in vivo studies for assessing the euthyroid status of 131 I-treated patients and provide an explanation for the discrepancy between clinical status and in vivo iodine tests.