Falsely Elevated Thyroid Hormone Levels Due to Anti-Sheep Antibody Interference in an Automated Electrochemiluminescent Immunoassay

Abstract

We describe a 43-year-old woman with falsely increased thyroxine (T₄) and triiodothyronine (T₃) concentrations (total and index values) using a competitive electrochemiluminescent immunoassay, due to human anti-sheep antibodies. The thyrotropin (TSH) concentration was within normal limits. When specimens were re-tested by an immunoassay utilizing mouse antibodies, the total T₄ and T₃ concentrations were within normal limits. Removal of IgG by protein G column chromatography resulted in normalization of total T₄ and T₃ concentrations. In contrast, a mouse IgG column failed to normalize the elevated total T₄ and T₃ concentrations. Other immunoassays utilizing mouse monoclonal antibodies and rabbit antisera were unaffected, indicating that the interference was anti-sheep antibodies and not heterophile antibodies. We believe this is the first report of human anti-sheep antibodies causing falsely increased total and free T₄ and T₃ serum concentrations in competitive immunoassays using sheep antisera. Clinicians need to be aware of this potential problem since inaccurate thyroid function tests can lead to inappropriate treatment decisions.