Acute Effect of Organic Binding of Iodine on the Iodide Concentrating Mechanism of the Thyroid Gland

The organic binding of radioiodine by the thyroid gland of the mouse was blocked within 1 minute by the intravenous injection of 1 mg of propylthiouracil (PTU). In mice fed a very low iodine diet the ratio of the radioiodide concentration in the thyroid gland to that in the serum (T/S) was reduced significantly by injection of 1.5 µg of iodide 15 minutes before intravenous PTU injection. The amount of iodine bound prior to PTU injection was only a small percentage of the total iodine in the thyroid gland. This reduction did not occur in mice with less active thyroid glands. However, in the latter mice, doses of 22.5 µg of iodide or more injected 90 minutes before PTU caused a small but systematic decrease in the T/S when compared at fixed serum iodide concentration with mice given PTU before the iodide. In addition, in these mice given both PTU and iodide the T/S was greater than in mice given iodide but no PTU. It is concluded that the organic binding of iodine causes a decrease in the T/S in mice with active thyroid glands. In addition, PTU increases the T/S in mice in which organic binding of iodine is blocked by iodide.