Homogeneous time-resolved fluoroimmunoassay of thyroxin in serum.

We describe a rapid, simple nonseparation fluoroimmunoassay for determination of thyroxin in serum. The assay is based on the labeling of thyroxin directly with a fluorescent europium chelate, the fluorescence of which is quenched on binding to an antithyroxin antibody. With the assay buffer we used, maximum quenching is 90%. The rapid achievement of equilibrium in the assay solution, regardless of the sequence of reagent additions, allows fast measurement of thyroxin. Precision was good (CV less than 5%) within the clinical range for total thyroxin (50-300 nmol/L), and results correlated well with those by a commercial radioimmunoassay.