FACTORS AFFECTING THE BINDING CAPACITY OF HUMAN ERYTHROCYTES FOR I131-LABELEDl-THYROXINE ANDl-TRIIODOTHYRONINE*†

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Abstract
THE purpose of this paper is to present further studies on the binding or uptake of I131-labeled thyroid hormones by human erythrocytes. Previous studies from our laboratory (1, 2) have shown that the presence of human plasma markedly inhibits the uptake of l-thyroxine and to a lesser extent l-triiodothyronine. This inhibition was considered to be due to the binding of l-thyroxine by the “thyroxine-binding protein” of plasma. The “thyroxine-binding protein” has not been isolated, but it has been shown by several investigators (3–6) that at physiologic concentrations thyroxine is principally associated with a protein which at pH 8.6 displays an electrophoretic mobility intermediate between those of alpha1 and alpha2 globulins. Data will be presented to show that human serum albumin inhibits the uptake by the red cell of radioactive l-thyroxine and l-triiodothyronine, but to a lesser degree than by an equal amount of human plasma, and that human gamma globulin causes no significant decrease in the uptake of radioactive l-thyroxine by the red cells.

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