Quantitation of thyroidal binding of iodide by compartmental analysis verified by an intravenous perchlorate discharge test

Compartmental analysis was applied to 123I-iodide uptake data to quantitate iodine binding in the human thyroid gland. The method allowed for arterio/venous differences in plasma tracer level and for an "instantaneous" phase of thyroid uptake. Results were checked by an intravenous perchlorate discharge test. Observations in eleven normal and untreated thyrotoxic subjects confirmed earlier findings that iodine binding takes place rapidly, the binding rate constant being much greater than the exit rate constant. A lower limit of 0.15 min-1 for the binding rate constant in the uninhibited gland was estimated from the observations in one subject who demonstrated a small perchlorate discharge. The method was used in the study of eight subjects with an intrinsic binding defect and of twenty-four thyrotoxic subjects being treated with 5 mg or 20 mg carbimazole, twice daily. Binding rate constants (range 0.003--0.105 min-1) were typically less than the exit rate constants (range 0.027--0.156 min-1), the net clearance of iodide ranging from 1.5 to 67.9% of the unidirectioinal clearance, compared to 72% in the uninhibited gland. The method proved useful in assessing the severity of an intrinsic binding defect and in the investigation of lack of response to antithyroid drug therapy.