EFFECT OF ADMINISTRATION OF HUMAN SERUM THYROXINE-BIND1NG GLOBULIN ON THE DISAPPEARANCE RATES OF THYROID HORMONES IN THE CHICKEN1

Abstract

The disappearance rates of intracardiac administered I¹³¹-labeled L-thyroxinc (T-4) and 3,5,3′-triiodo-L-thyronine (T-3) were measured by the whole body counting technique in young White Leghorn cockerels. The disappearance rate was expressed as a half-time for both a fast component (0-24) and a slow componet (24-54 hours). The half-times for both components were not different for the two hormones. When small amounts of a preparation of enriched human serum thyroxinebinding globulin (TBG) was mixed with radiothyroxine before administration of the labeled hormones, the half-time of the labeled T-4 in the first 24 hours was increased from 9.4 ± 0.5 to 14.2 ± 1.8 hours. TBG did not affect the disappearance rate of labeled triiodothyronine when the two were similarly injected nor did TBG affect the half-time of the slow componet of T-4 or T-3. Small amounts of TBG failed to influence the half-times of either of the two hormones if the TBG were administered 15 minutes or 24 hours after administration of the hormones, suggesting either that the hormone or the human serum protein was removed from the circulation before any combination of hormone and human serum protein could occur. The selective increase in half-time of T-4 by human TBG to a higher value than the half-time of T-3 in birds complements an earlier observation where the similar half-times of the two hormones in birds was attributed to a demonstrated absence of TBG in bird serum. The results also support the view that the relative potencies of the two hormones in different species may be in part, a reflection of rates of their distribution to the tissues.