Artifactual elevation of thyroid-stimulating hormone

A clinically euthyroid patient was found to have a normal serum thyroxine level and an elevated plasma thyrotropin (TSH) level measured by fluoroimmunoassay. Thyroid hormone therapy failed to suppress the TSH level. The TSH level was unresponsive to thyrotropin-releasing hormone (TRH) administration, alpha-subunits of pituitary glycoproteins were undetectable in her plasma, and imaging of the pituitary-hypothalamic region was normal. Measurement of TSH with an assay containing sheep antibody to TSH failed to reveal TSH in the patient's plasma. Addition of mouse IgG to the TSH fluoroimmunoassay reduced the patient's TSH to an undetectable level. These observations are consistent with a spurious elevation of TSH due to the presence of an anti-mouse antibody. Artifactual elevations of TSH have not been identified commonly, but this possibility should be considered when the TSH level is inappropriate for the apparent state of thyroid function.