Artefactual increase in serum thyrotropin concentration caused by heterophilic antibodies with specificity for IgG of the family Bovidea.

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Abstract
A 19-year-old white woman presented with symptoms compatible with mild hyperthyroidism and biochemical evidence suggestive of autonomous thyrotropin (TSH) secretion. Intensive investigation of the pituitary-thyroid axis suggested that the basal concentrations of TSH were artefactually increased owing to heterophilic antibodies in the patient's serum with a broad specificity for immunoglobulin class G of the family Bovidea. These heterophilic antibodies complexed with the ovine antisera to human thyrotropin that are used in the RIA system, in particular blocking the binding of TSH but also partly blocking interaction with the second antibody. When TSH was measured immunometrically or by an RIA with TSH-specific antisera of rabbit origin, the concentrations measured were within the appropriate reference intervals. The blocking effect can be overcome by including large quantities of non-TSH-specific ovine IgG in assay incubation mixtures. Interference of this type is generally not appreciated and its incidence is poorly characterized, but it may have implications for any method in which antibodies are used as reagents.