The performances of newer free thyroxine assays vs equilibrium dialysis.

Abstract

The performances of 5 different commercial kits for the measurement of serum FT4 concentration; Amerlex-M Free T4 RIA, Gammacoat 125I FT4 2 step RIA, Enzymune-test FT4, Amerlite FT4, Berilux FT4 were compared with equilibrium dialysis. All assays demonstrated FT4 values in good correlation with those measured by equilibrium dialysis (r = 0.84-0.89, p < 0.001). The intra-assay coefficients of variation were good and comparable. All methods yielded good separation of uncomplicated hypo/hyperthyroid patients from euthyroid controls. However, only equilibrium dialysis gave consistently normal results of FT4 values throughout pregnancy. All methods other than equilibrium dialysis gave falsely low values in majority of women in the 2nd and 3rd trimester of pregnancy.