Serum thyroxine-binding globulin: determination by competitive ligand-binding assay in thyroid disease and pregnancy

Serum thyroxine-binding globulin (TBG) concentrations were measured by competitive ligand-binding assay in normal subjects and patients with a variety of abnormalities of thyroid hormone production or TBG production. The mean serum TBG concentration in 73 normal adults was 3.4 +/- 0.6 mg/100 ml. No correlations with age, sex or serum thyroxine (T4) or triiodothyronine (T3) concentrations were found. Serum TBG concentrations were normal in patients with hyperthyroidism. They were significantly elevated in hypothyroid patients, and fell to normal with thyroxine treatment. In pregnant women, serum TBG concentrations were markedly increased, being 7.1 +/- 1.2 (mean +/- SD) mg/100 ml in women in their first trimester, 9.0 +/- 1.0 mg/100 ml in the second trimester and 8.9 +/- 1.6 mg/100 ml in the third trimester. There was a positive correlation between serum TBG concentration and both serum T4 and serum T3 concentrations in pregnant women. Competitive ligand-binding assay is a simple and reliable method for TBG asay and yields results similar to those of electrophoretic saturation techniques.