RESIN UPTAKE OF I131-TRIIODOTHYRONINE AS A TEST OF THYROID FUNCTION*

Abstract

A simple, reproducible, in vitro test of thyroid function was developed based upon the uptake of I131-triiodothyronine from serum by the anion exchange resin, IRA-400, in the formate cycle. One milliliter of serum which had been equilibrated with I131-triiodothyronine was added to 1 milliliter of resin in a calibrated tube. The mixture was shaken for ninety minutes and assayed in a well counter. After three washings with water the resin was counted again to determine the resin uptake. The findings (mean [plus or minus] standard deviation) were: (1) hypothyroidism, 27 [plus or minus] 3.3 per cent; (2) euthyroid subjects, 35 [plus or minus] 2.2 per cent; (3) thyro-toxicosis, 53 [plus or minus] 7.8 per cent. The elevated resin uptake values in active thyrotoxicosis fell after therapy. In pregnancy, the values were depressed to the hypothyroid level or lower. Contamination with organic and inorganic iodine did not interfere with the test.