Effect of Carbon Dioxide and Diphenylhydantoin on the Partition of Triiodothyronine labelled with Iodine-131 between the Red Cells and the Plasma Proteins

Abstract

THE uptake in vitro by the red blood cells of tri-iodothyronine labelled with iodine-131 is a useful method of evaluating thyroid function¹. It has been shown that under the conditions of the test ¹³¹I-triiodothyronine is distributed between the red cells, the α₂- and β-globulins and the binding sites on thyroxine binding protein not occupied by thyroxine (free T.B.P.), according to the relative binding powers of these substance; since the binding power of the red cells and the α₂- and β-globulins appears to be relatively constant, the test is an indirect measurement of free T.B.P. capacity². It is known that an increase in the carbon dioxide tension of the blood increases the proportional uptake of ¹³¹I-triiodothyronine by the red cells³. The uptake is also increased in patients who are being treated with diphenylhydantoin (phenytoin), an effect which may be reproduced when this substance is added to blood in vitro⁴. We have investigated the mechanisms whereby these agents modify the partition of ¹³¹I-triiodothyronine between the red cells and the plasma proteins.