Determination of free thyroxine in serum using column chromatography & radioimmunoassay.

A simple method for measurement of free thyroxine (FT4) levels in serum by adsorption chromatography and subsequent radioimmunoassay (RIA) is described. Assay sensitivity observed by this method was 1.2 pg/ml. Intra-assay variability was 11.2 per cent for FT4 concentrations of 7.9 pg/ml (n = 10), while interassay variability was 23.3, 9.7 and 12.9 per cent respectively for concentrations of 2.66, 12.06 and 23.96 pg/ml (n = 16). Serum FT4 concentration estimated by this method in 79 euthyroid patients was found to be 10.8 +/- 3.1 pg/ml. Values of FT4 obtained in hypothyroid and hyperthyroid patients were outside the normal range. FT4 values obtained by this method correlated well with the standard Liso-phase FT4 kit (r = 0.8) as well as free FT4 index (r = 0.93).