Radio immune assays (competition assay) used in diagnosing FDH :
Radio immune assays are useful in diagnosing FDH. In 1-step assay labelled T4 (the tracer) competes with serum T4 for a fixed number of anti-T4 antibody binding sites. Free harmone concentration is determined by the fractional occupancy of antibody binding sites. Equilibrium between T4 and its binding proteins is conserved during measurement, so that the amount of tracer displaced reflects the ’free’ rather than ’total’ hormone concentration. It is protein-dependent and is prone to under- or overestimate FT4.
In the 2 step assay wash step prior to tracer addition. It may reduce but not completely eliminate such interference. If the problem persists, hormone measurement following equilibrium dialysis (ED) remains the gold-standard for eliminating FT4 assay interference.