Falsely Lowered FT4 and Raised TSH Concentrations in a Patient with Hyperthyroidism and Human Anti-Mouse Monoclonal Antibodies

Human anti-mouse (HAMAs) are human that can cause interference in immunoassays, usually in assays using mousederived monoclonal antibodies. The incidence and extent of interference is variable, depending on the assay technique and whether a blocking agent such as mouse serum is added to the reagents. Usually, though not exclusively, the interference has been reported in sandwich-type thyroid stimulating hormone (TSH) assays.I-' HAMAs are one of a number of potential interferences, that include heterophilic antibodies.' anti-analyte antibodies and complement. 5 In the case reported by Fiad et al.' heterophilic caused positive interference with TSH, as well as free and total T4 and T3 assays. While spurious elevation of results due to anti-thyroid hormone is well recognized, the case reported by Fiad et al.' was unusual in that the positive interference was due to heterophilic antibodies. It occurred in various assays using antisera from mouse and sheep. This report describes the combination of a falsely lowered (rather than raised) free T4 concentration concurrent with a falsely increased TSH concentration. This combination, which was found to be due to HAM As, persisted during treatment and has not been reported previously.