Free Thyroxine Measured by Equilibrium Dialysis and Nine Immunoassays in Sera with Various Serum Thyroxine-binding Capacities

Despite the predominant role of thyrotropin measurements in the assessment of thyroid status, free thyroxine (FT4) measurements remain useful either when thyrotropin determination is not conclusive or when a diagnosis of thyroid disease must be confirmed (1). Because it represents only a minute fraction (0.02%) of total T4 (TT4), FT4 is more difficult to measure (2). Direct equilibrium dialysis (ED) methods are considered analytically accurate (3) and are the methods against which others are compared (4). Compared with ED, other FT4 immunoassays may show significant biases related to protein-bound T4 or to the serum T4-binding capacity (sBC: concentration × affinity of binding proteins) (4)(5)(6). We assume assays are calibrated to have roughly the same euthyroid range in samples with normal sBC, and we expect that markedly negative biases may be observed in samples with low sBC and that smaller positive biases may be observed in samples with high sBC (7). The aim of our study was to determine, in clinical samples from euthyroid patients classified into three groups as a function of their low, normal, or high sBC, the bias between FT4 measured with ED and that measured with nine frequently used immunoassays. We also studied the specificity of each assay method and the concordance of immunoassays with ED.FT4 was determined with the Nichols ED/RIA assay (Nichols Institute Diagnostics) and the following nine immunoassays: Elecsys (EL) from Roche Diagnostics, VIDAS (VD) from bioMerieux, Vitros ECi (VT) from Ortho-Clinical Diagnostics, GammaCoat 2-step RIA (GC) from DiaSorin, Immulite (IM) from Diagnostic Products Corporation (DPC), Nichols Advantage (AD), AxSYM (AX) from Abbott Diagnostic, ACS (AC) from Bayer Diagnostics, and AIA (AI) from Tosoh Bioscience. All assays were performed in compliance with the manufacturers’ instructions. The sBC was calculated …