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 [125 I]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 degrees C, while the uptake of [125 I]triiodothyronine was increased. There was a significant negative correlation between thyroxine-binding globulin concentration and triiodothyronine uptake in the heated serum samples and in euthyroid subjects.