High Total t4 à increased protein binding of the hormones
familial dysalbuminemic hyperthyroxinemia
presence of anti-T4 antibodies.
High TSH à presence of heterophilic Ab (in immunometric assays)
These human anti-mouse gamma globulins can bridge the two mouse monoclonal antibodies (solid phase antibody and signal antibody) and cause spuriously elevated readings for TSH [15,20].
Nonlinearity of TSH measurements with serial dilution of the patient’s serum strongly suggests assay interference.
Addition of nonimmune homologous mouse immunoglobulins has reduced this type of assay interference. Commercial assays exist for detecting human anti-mouse antibodies (HAMA).
Patients with substances interfering with TSH measurements are usually euthyroid, not thyrotoxic, and have normal free T4 and T3. Thus, tests for interference with the TSH measurement are rarely required.