The Establishment Of An Enzyme Immunoassay For Free Thyroxine (FT₄)

This one step competitive-type enzyme immunoassay for free thyroxine(FT4) involves simultaneous incubation of sample and thyroxine conjugated to horseradish peroxidase in antibody-coated microtiter plate at 37C for 45 minutes. The unlabelled analyte competes with labelled analyte for the limited number of available antibody binding sites thereby reducing the amount of labelled analyte bound to antibody. The amount of enzyme bound to the wall is,therefore,inversely related to the concentration of analyte in the patient sample or standard. FT4 values obtained with this assay for 154 samples,including 85 euthyroid subjects and 60 hyperthyroid and 9 hypothyroid patients. FT4 concentrations range from 7.27 to 25.75 pmol/L in 85 euthyroid subjects. The sensitivity of the assay is 1.67pmol/L. The measurable range of FT4 of this assay is 2-100 pmol/L. Intra-assay CVs range from 10.2%to 5.5%,interassay CVs from 13.6% to 6.3% at concentration of 8.69 to 41.26 pmol/L. For a reference radioimmunoassay,the correlation regression equation of present method is y= -1.943+1.097x,r = 0.893(n=101).