Relationship Between thein VitroUptake of131I-triiodothyronine by Erythrocytes and Its Binding by Serum Proteins in Thyroid Disease1

Abstract

The in vitro uptake of 161I-triiodothyronine (T3) by red blood cells (RBC) and its binding to serum proteins were estimated in 400 subjects (180 normal, 114 thyrotoxic and 106 hypothyroid) chosen at random. Total thyroxine-binding globulin (TBG) capacity and protein-bound iodine concentration (PBI) were also estimated in 89 subjects (31 normal, 35 thyrotoxic and 23 hypothyroid). The binding of T3 to serum proteins was shown to be more accurate (95.8%) than its uptake by RBC (89.8%) in the study of thyroid function, and both methods were more reliable in diagnosing hypothyroidism (95.3 and 89.6%) than thyrotoxicosis (92.1 and 82.4%). The uptake of T3 by RBC increases and the binding of T3 to serum proteins decreases as the concentration of thyroxine (T4) in plasma is artificially raised. The values for the uptake of T3 by RBC and its binding to serum proteins were demonstrated to be dependent on free TBG capacity. The maximum uptake of T3 by RBC and the lowest value for the binding of T3 by serum proteins ...