The Iodide Trapping

McGinty and Sharp (1) demonstrated that the thyroid glands of rats treated with propylthiouracil retained the ability to concentrate iodide ion but failed to bind the iodine in an organic form. VanderLaan and VanderLaan (2) and Stanley and Astwood (3) showed also that thyroidal iodide, "trapped" under the influence of the antithyroid drugs, could be rapidly discharged from the gland by the administration of thiocyanate ion. Iodine accumulation by the thyroid has therefore been considered a two-step function; the first is concerned with the concentration (trapping) of iodide at a level higher than that in the circulating plasma, while the second, in itself a complicated process composed of several chemical stages, is represented by organic binding of the trapped iodide. Several authors have inquired into the kinetics of thyroidal iodide trapping and binding (4-6). On the assumption that equilibration of plasma and thyroidal inorganic iodide131 (iodide trapping) is instantaneous following intravenous administration of 1131, Ingbar (4, 5) calculated that thyroidal binding of trapped iodide proceeded at average rates of 1% per cent per minute in euthyroid subjects and 6% per cent per minute in patients with Graves' disease. Wollman (6) assumed that the plasma iodide131 concentration remained constant or decreased at a single exponential rate and considered the sequelae of several theoretical alternatives for the relative rates of trapping and binding. In the present study an analysis is presented which permits evaluation of the rates of trapping and binding from observed data without assumptions regarding either process or restrictions in the behaviour of the plasma iodide131 concentration. The results of the experimental investigations indicate an extremely rapid binding of trapped iodide in the untreated gland and lead to the conclusion that trapping rather than binding is the rate-limiting step of thyroidal iodine accumulation. TECHNICAL METHODS