Studies on the accuracy of a quantitative kit radioimmunoassay for free thyroxine

Experiments designed to determine the accuracy of the Corning Immophase Free T4 assay revealed that there was a marked perturbation of thyroxine binding to the serum proteins during the assay; binding of 125I-T4 to the immobilised T4 antibody in the absence of merthiolate increased considerably with buffer dilution of serum; and the free thyroxine concentration declined significantly during pregnancy. These changes in measured fT4 were not observed with an equilibrium dialysis-radioimmunoassay procedure. The assay was precise, easily performed, and as effective as the free thyroxine index (FTI) in diagnosing thyroid disease. We conclude that the assay does not provide an accurate quantitative estimation of serum fT4 concentration in samples with elevated TBG concentration and that the kinetic principles on which the assay is based are altered as serum binding-protein concentration is reduced (serum dilution). Both FTI and fT4 (Corning) data in pregnant patients should be interpreted with caution and with reference to clinical symptomatology and other thyroid function tests.