Degradation of Thyroxine Confounded by Thyroidal Recycling of Radioactive Iodine 1

Abstract

Summary

Data are presented to show that measurements of T₄ degradation rate in bob-white quail, when using radioactive T₄ as a tracer, are confounded by recycling through the thyroid of I^- metabolized from the labeled hormone. The effects of this can be minimized by (1) blocking the thyroid I^- trap with SCN^-, (2) taking the blood samples over a short period of time after equilibration of the injected radioactive T₄ or (3) mathematically correcting for recycling in data collected over an extended period. The biological half-life for the disappearance of injected radioactive thyroxine in two experiments, including four groups of birds, ranged from 6.6 to 8.6 hr.