Abstract
We describe a two-step radioimmunoassay procedure (RIA) for the measurement of free thyroxine (fT4) in human serum. A commercial antibody with the affinity of 5.5 × 109 L/M was used for this study. The anti-T4 antibody was immobilized on the inner walls of the polystyrene tubes by passive adsorption through normal rabbit IgG and anti-rabbit IgG as immunobridges. The method developed uses very small amounts of antibody with minimum dilution of the sample. The assay covers the range of 0 to 87.5 pmol/L with sensitivity of 0.9 pmol/L. The intra-assay precisions were 6.3% and 7% at 6.7 and 20.7 pmol/L, respectively, for 15 replicates. The inter-assay precision at 6.4 and 24.4 pmol/L were 13% and 11.6%, respectively. The normal range established by analyzing 54 healthy volunteers was 15.3–24.7 pmol/L and that of 69 ambulatory subjects was 14.8–23.3 pmol/L. Progressive dilution of euthyroid and hypothyroid samples up to 50-fold yielded virtually constant values, whereas a registered decrease in free T4 was observed with hyperthyroid samples. Free T4 estimated for 75 samples, including samples from 16 pregnant female subjects, correlated well with the values obtained by a reputed commercial kit with the linear correlation coefficient of 0.84.
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