Misleading FT4 measurement: Assay-dependent antibody interference

Introduction: Commonly used free thyroxine (FT4) immunoassays can be falsely elevated due to interference causing misinterpreted thyroid function. We present two cases with high FT4 concentrations due to antibody interference. This study's aim was to investigate the source of the FT4 immunoassay interference and possibility of its removal by two different techniques in order to correct the discrepancy between obtained FT4 values and the patient's clinical status. Materials and methods: Two patients presented at their general practitioners' with elevated FT4 concentrations in combination with a normal and increased thyroid stimulating hormone (TSH) concentrations. Clinical symptoms differed between patients but did not correspond with the hyperthyroid status suggested by the laboratory results. FT4 concentrations from both patients were measured on four common commercial immunoassays and the dialysis method before and after treatment with heterophilic blocking tubes and protein A/G. Results: Removal of interfering antibodies using protein A/G resulted in normal FT4 concentrations. Conclusion: This report illustrates falsely elevated FT4 concentrations due to assay interference on the Immulite immunoassay analyser caused by heterophilic antibodies, which were eliminated by protein A/G treatment. We point out the importance of a close collaboration between doctors and the laboratory to avoid unnecessary clinical intervention.