Quantitative Studies of Iodine Metabolism After Long-Term Treatment of Thyrotoxicosis with Antithyroid Drugs

Abstract

Seventeen thyrotoxic patients were treated for 18 months with methylthiouracil or carbimazole. Quantitative studies of iodine metabolism were performed on the last day of therapy and at regular intervals thereafter for up to 1 yr. While receiving treatment, levels of plasma inorganic iodine (PII) were normal, the absolute iodine uptake (AIU) was high, but the serum protein-bound iodine (PBI) was low. The discrepancy between the last 2 parameters suggested that some of the iodide taken up by the gland was not being effectively converted to thyroid hormone. After discontinuing treatment, the PII fell to levels characteristic of iodine deficiency, but because there was a rise in the thyroidal iodide clearance rate the AIU remained at high normal levels. It is suggested that iodide taken up by the gland was stored in order to replenish the intrathyroidal iodine stores, which had been depleted during drug administration. A further 8 patients received thyroxine supplements during antithyroid drug administration. Of the 25 patients studied 7 relapsed. Iodine deficiency produced by prolonged antithyroid drug administration may delay relapse after discontinuing therapy.