TRIIODOTHYRONINE BINDING AFFINITY FOR THYROXINE-BINDING GLOBULIN IN SERUM FROM NORMAL SUBJECTS AND FROM HYPERTHYROID, HYPOTHYROID AND PREGNANT PATIENTS

Abstract

The binding of 3,5,3'-triiodothyronine (T₃) to thyroxine-binding globulin (TBG) was quantitated in pooled serum from normal, hyperthyroid, hypothyroid and pregnant subjects. To aliquots of each pool was added a small quantity of ¹³¹I-T₃ and varying concentrations of unlabelled T₃ or thyroxine (T₄). Following countercurrent electrophoresis in both barbital and trismaleate buffers at pH 8.6, the concentration of ¹³¹I-T₃ bound to TBG was calculated and the displacing effect of unlabelled T₃ and T₄ compared. The displacement of ¹³¹I-T₃ from TBG was greater with T₄ than with T₃ when normal, hypothyroid and pregnancy sera were electrophoresed in barbital buffer (which inhibits thyroid-binding prealbumin (TBPA) binding of T₄). The same findings are seen when hyperthyroid serum which is deficient in TBPA is electrophoresed in trismaleate buffer (which does not inhibit TBPA binding of T₄). The observed data indicate that T₄ has a greater binding affinity for TBG than has T₃ but only displaces T₃ to significant degree when TBG is saturated and when TBPA binding of T₄ is reduced or inhibited. TBG binding capacities for T₃ have been derived for each of the 4 clinical groups and appear to be less than corresponding binding capacities for T₄.