Falsely High Serum Free Triiodothyronine and Free Thyroxine Concentrations Attributable to Anti-Diiodothyronine and Anti-Triiodothyronine Antibodies

Abstract

We observed a patient with markedly increased free triiodothyronine (FT3) and free thyroxine (FT4) concentrations measured on a Vitros ECi analyzer (Ortho Clinical Diagnostics). The patient was a hospitalized 42-year-old woman with lupus erythematosus who appeared euthyroid and had normal thyroid-stimulating hormone (TSH) concentrations (Table 1 in the Data Supplement that accompanies the online version of this letter at http://www.clinchem.org/content/vol51/issue6/). Thyroid peroxidase antibody and thyroglobulin antibody were <0.3 kilounits/L, and rheumatoid factor was <6.0 kilounits/L. We suspected interference from heterophilic antibodies (1)(2), but our experiments suggested interference from antibodies to diiodothyronine (T2), T3, or their conjugates, as have been described (3)(4)(5). The procedures in this study were in accordance with the Helsinki Declaration of 1975 and the subsequent 1996 amendments. In contrast to results of the Vitros ECi FT3 and FT4 assays, which use solid phases with T2- and T3-gelatin, respectively, and labeled sheep antibodies, FT3 and FT4 were normal by the Elecsys assays (Roche Diagnostics), which use biotinylated antibodies in a one-step …