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

¹ Laboratoire Universitaire de Biophysique, Unité d’Analyses Endocriniennes, Université Louis Pasteur (ULP)/Centre National de la Recherche Scientifique (CNRS) Unité Mixte de Recherche (UMR) 7004, Faculté de Médecine, 67085 Strasbourg Cedex, France

² Service Central de Médecine Nucléaire, Hôpital Salengro, Centre Hospitalo-Universitaire Regional (CHRU), 59037 Lille Cedex, France

^aaddress correspondence to this author at: Institut de Physique Biologique, Faculté de Médecine, F-67085 Strasbourg Cedex, France; fax 33-3-90-24-40-57, e-mail sapin@ipb.u-strasbg.fr

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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 x 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 bioMérieux, 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’ . . . [Full Text of this Article]

The following articles in journals at HighWire Press have cited this article:

				B. Yue, A. L. Rockwood, T. Sandrock, S. L. La'ulu, M. M. Kushnir, and A. W. Meikle Free Thyroid Hormones in Serum by Direct Equilibrium Dialysis and Online Solid-Phase Extraction-Liquid Chromatography/Tandem Mass Spectrometry Clin. Chem., April 1, 2008; 54(4): 642 - 651. [Abstract] [Full Text] [PDF]