Stability of radiothyroxine plasma disappearance curve despite catharsis and unblocked thyroidal uptake of radioiodide.

Plasma radioactivity was measured over 21 days after an intravenous injection of 50 muCi of 125I-T4 in eight normal men. No thyroid-blocking medication was given. Four subjects (castor oil group) received 30 ml of castor oil on each of Days 13, 14, and 15, while the other four subjects (control group) were studied without medication. After A 5-day equilibration period, plasma 125I-T4 was measured on Days 5-13 in order to calculate the disappearance curve for each subject and to derive the mean for each experimental group. The curves were then extrapolated to Day 21. Measured radioactivity did not depart significantly from the extrapolated line, either during the castor oil period (Days 14, 15, and 16) or during the recovery period (Days 17, 19, and 21). The castor oil, therefore, had no observable effect on the clearance of plasma radioactivity. None of the subjects had a late increase in plasma radioactivity to suggest recirculation of radioiodide or buildup of iodoproteins. In normal subjects, radiothyroxine plasma levels up to 21 days are not significantly affected by short-term catharsis or by failure to block thyroidal radioiodide uptake.