The effect of plasma on the in vitro uptake or binding by human red cells of radioactive I-131 labeled L-thyroxine and L-triiodothyronine.

Thyroxine is bound to a serum protein which appears in the electrophoretogram just ahead of alpha-2 globulin (1-4). Triiodothyronine also associates with this thyroxine binding globulin (T.B.G.) but can be displaced readily with thyroxine -while triiodothyronine will not readily displace thyroxine. Deiss, Albright, and Larson (5) suggested that the firm binding of thyroxine to T.B.G. might account for the slower rate of disappearance of thyroxine from the plasma as compared to triiodothyronine. The purpose of this paper is to report studies on the in vitro uptake or binding by human red cells of radioactive I-131 labeled sodium iodide, human serum albumin; triiodothyronine, and thyroxine. We propose to show that the binding of L-thyroxine by plasma is one of the limiting factors in the uptake or binding of this substance by the human red cell. The uptake or binding of L-triiodothyronine is limited to a lesser degree by plasma binding and is consistently greater than the uptake or binding of L-thyroxine.