Interference in thyrotropin (TSH) determination: falsely elevated TSH values in a transplanted patient

We report a falsely elevated TSH value as determined by an immunoradiometric assay. The patient, a thyroidectomized woman on substitutive therapy, underwent a kidney transplantation. She was treated for transplant rejection with OKT 3 (Orthoclone), a mouse monoclonal antibody. The discrepancy between the TSH value (TSH-Riabead II, Abbott) and the thyroid hormones level led us to suspect the presence of antimouse antibodies in the serum of this patient. Indeed, human immunoglobulins directed against mouse IgG2a and IgG2b were detected. We repeated the TSH determination in the presence of normal mouse serum (NMS) and obtained a true TSH value. Upon fractionation of the serum with protein-A chromatography, the falsely elevated TSH immunoreactivity was detected in the IgG fraction.