A simple ligand-binding assay for thyroxine-binding globulin on reusable Sephadex columns

A method for the assay of thyroxine-binding globulin on reusable Sephadex G-25 (fine) columns is described. It depends upon elution by diluted iodothyronine-free serum of protein-bound [ 125I]thyroxine from the columns under conditions where binding to thyroxine-binding prealbumin and albumin are abolished. It is simple, rapid and precise and permits determinations in large numbers of samples. Values (mg/l; mean ± S.D.) were: normals 31.6 ± 5.4, hyperthyroid 28.3 ± 4.8, hypothyroid 40.6 ± 7.5, oral contraceptives 40.1 ± 6.8, pregnant 50.3 ± 5.4, cirrhotics 20.7 ± 4.3. Concentrations were reduced in serum heated at 56°C, while the uptake of [ 125l]triiodothyronine was increased. There was a significant negative correlation between thyroxine-binding globulin concentration and triiodothyronine uptake in the heated serum samples and in eu thyroid subjects.