Defects in intrathyroidal binding of iodine and the perchlorate discharge test

Abstract

Abstract. The kinetics of [123I]iodide uptake were studied when organification of iodine by the thyroid gland was normal and when this binding function was diminished by drugs or disease. Each study was terminated by a sodium perchlorate discharge test (300–600 mg iv) at 60 min or, in some cases, 10–30 min. The results confirmed that binding takes place rapidly in the uninhibited gland with the binding rate constant being at least 0.150 min−1. Discharge from the uninhibited gland is less than 3.5% of the gland uptake when perchlorate is given 60 min after the radioiodide. Subjects with an intrinsic binding defect manifested discharges of 11% or greater of the 60 min uptake and the estimated binding rate constants ranged from 0.003–0.057 min−1. Thyrotoxic subjects receiving 5 mg carbimazole twice daily manifested discharges ranging from 5.4–64.2%, and in those receiving 20 mg twice daily the observed discharges were 67.6–94.6% of the 60 min uptake. The study shows that a correctly performed perchlorate discharge test will detect minimal inhibition of iodine binding. An important factor is the duration of the follow-up period after perchlorate is given. In some of the cases studied discharge was not complete until 60 min after the perchlorate.