UNDERESTIMATES OF SERUM-FREE THYROXINE (T4) CONCENTRATIONS BY FREE T4IMMUNOASSAYS

Different free T-4 (FT4) assays often give different FT4 measurements, and conflicting measurements have been striking in nonthyroidal illne ss. Because FT4 immunoassays depend upon serum protein-bound T-4 (PBT4 ) dissociation to stabilize the FT4 concentration during assay perturb ations, interassay differences in perturbations combined with variatio n in serum PBT4 concentrations could produce discordant FT4 measuremen ts. This study examined the effects of PBT4 on FT4 measurements obtain ed by direct immunoassay methods. Standard solutions with constant FT4 levels and varying PBT4 concentrations were prepared and analyzed by direct equilibrium dialysis, two-step immunoextraction, one-step label ed T-4 antibody, and one-step labeled T-4 analog FT4 methods. Direct e quilibrium dialysis results were independent of PBT4 concentrations an d gave correct measurements of serum FT4 when the PBT4 concentration w as above 8 nmol/L or 0.6 mu g/dL, but were PBT4 dependent and underest imated serum FT4 at lower PBT4 concentrations. The other three methods were PBT4 dependent and variably underestimated serum FT4 at all leve ls of PBT4 up to 256 nmol/L (19.9 mu g/dL), the highest level studied. Thus, PBT4-dependent underestimates of serum FT4 occurred with all fo ur methods, whereas the measured FT4 level at each PBT4 concentration varied widely between methods. A serum PBT4 dependent bias causes disc ordant FT4 measurements and probably explains the observed underestima tes of FT4 in nonthyroidal illness.