Heterophilic serum antibodies: a cause for falsely elevated serum thyrotropin levels.

We report two cases of euthyroid patients with inappropriately elevated serum levels of thyrotropin (TSH) in whom investigation led to the detection of heterophilic antibodies. In addition, we compare the performance of two TSH immunometric assay systems in this setting. Even when monoclonal assay systems are used, TSH results must be critically interpreted when they are at variance with clinical and other biochemical findings.