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

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-T3and varying concentrations of unlabelled T3or thyroxine (T4). Following countercurrent electrophoresis in both barbital and trismaleate buffers at pH 8.6, the concentration of 131I-T3bound to TBG was calculated and the displacing effect of unlabelled T3and T4compared. The displacement of 131I-T3from TBG was greater with T4than with T3when 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 T4has a greater binding affinity for TBG than has T3but only displaces T3to significant degree when TBG is saturated and when TBPA binding of T4is reduced or inhibited. TBG binding capacities for T3have been derived for each of the 4 clinical groups and appear to be less than corresponding binding capacities for T4.