Assay of serum thyroxine by competitive protein binding: reuse of disposable sephadex columns

A method for the assay of total serum thyroxine on reusable Sephadex G-25 fine columns, obtained from the Ames Tetralute kit, and competitive protein binding with TBG is described. The columns are regenerated with diluted plasma and may be used for at least 3 months. The procedure is rapid and simple and requires no extraction, centrifugation or evaporation. Recovery of added thyroxine from serum is essentially complete. The between-assay and within-assay coefficients of variation are about 5.5%. Correlation with values obtained using the Tetralute kit was good (r = 0.988). Values in 75 euthyroid subjects were normally distributed and therefore the normal range was taken as the mean +/- 2 SD or 4.6-10.4 mug/100 ml.