A 63-year-old woman with papillary thyroid carcinoma showed increased TSH values (measured by two-site immunometric assay kit). X-ray and endocrinological examinations did not prove the coexistence of pituitary neoplasm. TSH values which were measured by using an another TSH assay system (TSH RIABEAD II assay kit) revealed normal range. Measured TSH values were reduced by adding mouse IgG into serum. These findings suggest that substances in serum, which can link reagent mouse antibody in the assay system, may give falsely increased TSH values. We should be aware that two-site immunometric assay may give falsely increased hormone values due to the appearance of substances against anti-animal antibodies in serum.