Kinetics of radioiodine released from prelabelled thyroid gland in vivo: influence of propylthiouracil.

The kinetics of free and hormone bound blood iodine after stimulation with endogenous thyroid stimulating hormone (TSH) are not satisfactorily characterized. We studied these kinetics in mice injected with 125I and thyroxine. In control mice, the injected 125I is organified within the thyroid and incorporated into thyroid hormones, whereas in mice treated with the thyreostatic drug propylthiouracil (PTU), most 125I remains inorganic, since the thyroperoxidase activity is inhibited by PTU. We found that during blockade of TSH secretion by means of thyroxine, blood 125I activity was significantly higher in PTU-treated animals than in controls, indicating that PTU impaired thyroidal uptake of 125I. On the fourth day after the thyroxine load, the blockade of TSH secretion vanished. This caused the blood 125I activity to increase markedly. The increase of blood 125I was as high in PTU-treated animals as in controls. After the peak, blood 125I was cleared according to first order kinetics, with a half-time of 0.72 days (= 17.3 hours) in PTU-treated animals and of 6.3 days in controls (P less than 0.001). It is suggested (1) that PTU impairs thyroidal uptake of iodide, (2) that endogenous TSH stimulates release from the thyroid of inorganic iodide as well as of thyroid hormones, and (3) that inorganic iodide released by the thyroid has a much shorter biological half-life than hormone-bound iodine.