Kinetics of the human thyroid trap: effects of iodide, thyrotropin, and propylthiouracil.

Effects on the thyroidal pertechnetate trap of iodide, thyrotropin (TSH), and propylthiouracil (PTU), compared with duplicated control studies, were assessed in normal subjects using i.v. [Tc99m] pertechnetate, a multicrystal scintillation camera, and a compartmental model. Sodium iodide (1 g), administered orally on two occasions, 2 wk apart, caused an early drop in plasma clearance into the follicular cell (p less than 0.05), with later return to normal clearance 1 wk after the second NaI dose. In this later study, exit from the colloid was elevated (p less than 0.01). Plasma equivalent volume of the "colloid" compartment was reduced in both postiodine studies (p less than 0.05). Thyrotropin, 10 units intramuscularly, was followed by no significant changes in trap parameters at 2 hr. At 24 hr, plasma clearance had doubled (p less than 0.05), and the plasma equivalent "colloid" volume had tripled (p less than 0.01). Propylthiouracil was given as a single 1 g dose 1 hr before a trapping study followed by 200 mg PTU every 8 hr for 1 wk. The first dose resulted in apparent reduction in all of the rate constants for transport across the basal and apical thyroid follicular cell membranes; these rates returned toward control levels after 1 wk. The plasma equivalent "follicular cell" volume was reduced to 66% of controls levels (p less than 0.025) after 1 wk PTU. These effects must be taken into account in the interpretation of studies of the trap based on PTU pretreatment to inhibit organification.