Using double-antibody coated tubes

a solid-phase radioimmunoassay for free thyroxine (FT4) in serum has been developed. The antibody immobilized onto the surface of the coated plastic tubes competed for binding with FT4 or 125I labelled T4-analogue. Whole test was completed within 2h. The detectable range was 0.12 to 75pmol/L FT4 in serum. The mean coefficients of variation within and between assays were 1.7%-4.4% and 3.2%-9.1%

respectively. The non-specific binding was less than 1%. This new assay was compared with a commercial radioimmunoassay kit for FT4 (labelled antibody). A linear regression analysis yielded a slop of 1.08

an intercept of 2.3 and a correlation coefficient of 0.85. Most of the analytical indices are better than the routine RIA procedure. Therefore it appears to be a simple

precise and accurate new assay method.