Preparation of 125I labeled human thyroxine binding alpha globulin and its turnover in normal and hypothyroid subjects

Abstract

A protein with the electrophoretic, immunologic, and hormone binding properties of thyroxine binding globulin (TBG) has been prepared from human plasma and labeled with radioiodine (125I) by an enzymatic method of iodination. The [125I]TBG retained the electrophoretic and immunologic characteristics of unlabeled TBG but exhibited a partial loss of thyroxine binding activity, as assessed by affinity chromatography. The in vivo behavior of [125I]TBG was studied in six euthyroid subjects (controls) with normal serum levels of TBG as measured both by radioimmunoassay and by determination of maximal T4 binding capacity and in four male patients with untreated primary hypothyroidism, three of whom had elevated serum TBG. The half time of the final slope of the plasma disappearance curve averaged 5.0 days ±1.2 (SD) in the controls and ranged from 3.9 to 10.9 days in the hypothyroid patients. The distribution volume was similar in the two groups, 6.7±1.3 vs. 7.1±2.1 liters. The catabolic clearance rate averaged 0.99±0.33 liters plasma/24 h in the controls and 0.92±0.46 in the hypothyroids. The absolute turnover rate of TBG, calculated from the catabolic clearance rate multiplied by the serum concentration of radioimmunoassayable TBG, averaged 17.8±2.1 mg/day in the controls and ranged from 14.8 to 33.2 mg/day in the hypothyroids. Among the entire group of subjects there was no correlation between the serum TBG concentration and the absolute turnover rate of TBG.