Unexpected differences in early thyroidal trapping of iodide and pertechnetate.

Compartmental analysis was applied to simultaneously acquired 132-I-iodide and 99mTc-pertechnetate thyroid uptake data. The method allowed for arterio/venous differences in plasma tracer level and for an 'instantaneous' phase of thyroid uptake. Observations in four thyrotoxic patients, before and during antithyroid drug therapy, revealed greater 'instantaneous' uptake of TcO4-. The results also revealed that 'instantaneous' uptake of both I- and TcO4- may increase over the first six months of drug therapy. These findings could not be explained by the estimates of unidirectional clearance which were greater for I- and varied little during the early stages of drug therapy.