Suppression of Release of Radioactive Iodine as a Test of Thyroid Function*

Abstract

Sixteen euthyroid and 19 thyrotoxic subjects were given a tracer dose of radioactive iodine and the 24-hour thyroidal I¹³¹ uptake was determined. Immediately after the 24-hour uptake was measured, the subjects were given 25 μg of triiodothyronine every 8 hours and 30 mg of methimazole every 6 hours. Thyroidal radioactivity was determined periodically thereafter. Seven days after administration of the tracer, 92 (±1.8) per cent of the labeled iodine remained in the thyroids of the euthroid subjects whereas only 49.1 (±3.8) per cent was retained by the thyrotoxic patients. The difference between the two groups was sufficiently great to be of diagnostic value. Suppression, by thyroid hormone, of the release rather than the uptake of I¹³¹ offers several practical advantages as a diagnostic test.