Thyroglobulin autoantibody levels below the cut-off for positivity can interfere with thyroglobulin measurement.

Antithyroglobulin antibodies can interfere with the measurement of thyroglobulin yielding spuriously high or low levels depending on the method used. Interference is unrelated to the antibody concentration and can occur at very low concentrations. We report a patient in whom antithyroglobulin antibodies below the cut-off for positivity nearly led to an incorrect diagnosis of thyrotoxicosis factitia.