Because it represents only a minute fraction of the total serum T4, measuring free thyroxine (FT4), i.e., T4 not bound to binding proteins, remains difficult.The aim of the authors' study was to compare FT4 concentrations in sera from patients with nonthyroidal illnesses, measured with three recent nonisotopic automated techniques and with three manual RIAs: two immunoextn. assays (one two-step and one one-step assay), and a direct equilibrium dialysis (ED) assay considered as the reference method.The patient panel consisted of 20 hospitalized subjects on the seventh day following a bone marrow transplant, an example of severe acute illness.Two automated FT4 methods were labeled-antibody immunoassays: the electrochemiluminescent Elecsys FT4 assay from Roche Diagnostics performed on the 2010 analyzer; and the enhanced chemiluminescent Vitros ECI FT4 assay from Ortho-Clin. Diagnostics.The third automated assay was a non-isotope-labeled analog technique: ADVIA Centaur FrT4 from Bayer Diagnostics.The two-step RIA was the GammaCoat FT4 from DiaSorin, and the one-step RIA was the labeled antibody kit Amerlex-MAB FT4 from Ortho-Clin. Diagnostics.The manual direct ED technique was the Nichols FT4 method (Nichols Institute Diagnostics).In addition to the FT4 assays, TSH was measured using four third-generation automated immunometric assays: the Immulite TSH-3G from Diagnostic Products Corporation performed on the Immulite analyzer; the Elecsys TSH from Roche Diagnostics performed on the Elecsys 2010 analyzer; the Vitros ECI TSH from Ortho-Clin. Diagnostics; and the ADVIA Centaur TSH-3 from Bayer Diagnostics.Total T4 was measured with the Elecsys T4 assay from Roche Diagnostics performed on the 2010 analyzer.Despite a neg. bias of the Vitros ECI TSH at concentrations <0.2 mIU/L, the concordance between all TSH methods was good in bone marrow-transplanted patients.The FT4 concentrations measured with recent automated methods were not equivalentAmong the three automated methods studied, which may gain widespread use, only the Vitros ECI showed a good concordance with the ED and GammaCoat kits, which frequently gave increased values and which are known to be tech. reliable assays.However, patients with nonthyroidal illnesses, such as those of this study, are not considered as hyperthyroid but as presenting with euthyroid sick syndrome.They do not benefit from treatment directed at normalizing FT4.FT4 results within the reference intervals were obtained in 85% of the patients with the Elecsys assay and in 80% of the patients with the ADVIA Centaur and Amerlex-MAB assays.If the "quality" of a FT4 method, in this situation, is judged on a "to treat or not to treat" basis, FT4 assay methods that more frequently produce values within the reference intervals, such as the Elecsys, ADVIA Centaur, and Amerlex-MAB, should be preferred.More than ever it is necessary to have a good knowledge of the FT4 method used to correctly interpret the results of thyroid function tests in patients with nonthyroidal illnesses.