Concentration of 35S-propylthiouracil by the thyroid gland and its relationship to anion trapping mechanism

Abstract Propylthiouracil is concentrated by the thyroid gland in rats and man. Four 35 S compounds were demonstrated in the rat and human thyroid by thin-layer chromatography: sulphate, intact propylthiouracil, an unknown metabolite X and origin 35 S activity which is protein bound. Unbound 35 S compounds in plasma include propylthiouracil, the glucuronide conjugate of PTU, sulphate, and X. The thyroid concentration rather than the plasma level of propylthiouracil determines the duration of inhibition of organic binding of iodine in the thyroid. The effect of perchlorate and iodide on the thyroid concentrating mechanism for 35 S-propylthiouracil and 125 I-iodide was studied. The level of perchlorate and iodide which almost completely blocked the concentration of 125 I-iodide also partially reduced the concentration of 35 S-propylthiouracil. Iodide but not perchlorate decreased the rate of metabolism of 35 S-propylthiouracil in the thyroid gland.