Defects in intrathyroidal binding of iodine and the perchlorate discharge test

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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 i.v.) at 60 min or, in some cases, 10-30 min. 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. 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.

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