The Measurement of the Iodine-accumulating Function of the Human Thyroid Gland.

At least two distinct mechanisms appear to be responsible for accumulation of iodine by the thyroid gland. The first, called the "iodide trap" by Vanderlaan and Vanderlaan (1), involves the concentration, presumably within the thyroid cells, of inorganic iodide obtained from the circulating blood. The second, called hormonal synthesis, involves the utilization of the trapped iodide for the synthesis of organic iodine compounds (chiefly diiodotyrosine and thyroxine) which are then stored in the colloid in thyroglobulin. Vanderlaan and Vanderlaan (1), and Taurog, Chaikoff and Feller (2) have shown in the rat that, even after hormonal synthesis has been blocked by an antithyroid drug such as propylthiouracil, the thyroid concentrates relatively large quantities of radioiodine. The radioiodine so concentrated remains in the form of iodide and Taurog and associates have shown that it leaves the thyroid at a rate which parallels the decrease of radioiodine in the blood, suggesting that an equilibrium exists between the two. This ability to concentrate radioiodine can be promptly abolished by the administration of thiocyanate, which thus appears to act as a specific poison to the iodide trap. The iodide-concentrating function has been studied in the human thyroid by Stanley and Astwood (3), who gave tracer doses of I'sl' some hours after administration of a dose of mercaptoimidazol or propylthiouracil. Significant concentration of radioiodine occurred in the thyroid, reaching a peak in one to three hours, thereafter decreasing and largely disappearing within 24 hours. Administration of potassium thiocyanate after the curve of iodine concentration in the thyroid had reached a maximum was followed