PATHWAYS OF ELIMINATION OF C14-LABELED THYROXINE IN THE RAT1

Abstract

Introduction TAUROG, Briggs, and Chaikoff (1951) using I¹³¹-L-thyroxine demonstrated the presence of an unknown metabolic product, in addition to thyroxine, in the bile of rats. These investigators (1952) then reported that the thyroxine-containing derivative may be a glucuronide, but not a peptide. They further concluded that the conjugate is the primary metabolic product of endogenous thyroxine, and that the bile is the chief pathway of elimination for this hormone. The present study is concerned with the elimination of DL-thyroxine-1-C¹⁴ in the bile, urine, and expired air. Further analysis of the bile has indicated that the thyroxine conjugate is not a peptide. methods DL-thyroxine-1-C¹⁴ used was that synthesized by Wang, Hummel, and Winnick (1952)⁴ Its specific activity was 5.3 μc per mg. In all radioactivity measurements, the observed counts per minute were converted into equivalent μgm. of thyroxine, by reference to the known specific activity of the administered C¹⁴-thyroxine.