IN VIVO STUDIES ON THYROXINE AND TRIIODOTHYRONINE METABOLISM IN THE RAT

Abstract

ABSTRACT The reciprocal effects of loading amounts of thyroxine (T4) or triiodothyronine (T3) on the in vivo deiodination of their 125I-labelled isotopes by the rat were studied. It was found that the deiodination of a tracer amount of [125I]T3 as measured by the 24-h urinary 125iodide was significantly decreased after administration of a loading dose of non-radioactive T4. A similar decrease in deiodination of [125I]T3 was observed following the injection of a T3 load. In experiments in which the deiodination of [125I]T4 was assessed, the effects on this of loading doses of non-radioactive T4 or T3 differed; while stable T4 induced a significant decrease in deiodination of [125I]T4, stable T3 was without effect. The data is in agreement with the view that T4 and T3 share tissue deiodinating sites and that in such a system T4 appears to have a stronger avidity for those sites.