POSITIVE TEST FOR ANTITHYROGLOBULIN ANTIBODIES DUE TO ADMINISTRATION OF IMMUNOGLOBULIN REPLACEMENT THERAPY IN A PATIENT WITH THYROID CANCER.

Objective: Thyroglobulin (Tg) is used as a tumor marker to monitor differentiated thyroid cancer progression and recurrence. However, Tg measured by standard immunoassay (IMA) is not a reliable marker in the presence of anti-Tg antibodies (TgAbs) due to interference that may result in either false-positive or false-negative results. TgAbs levels can be high due to thyroid cancer and also exogenous immunoglobulin (Ig) administration, thus making it difficult to identify differentiated thyroid cancer recurrence.

Methods: We present an example of elevated TgAbs due to subcutaneous Ig (SCIg) administration in a patient with thyroid cancer.

Results: A 57-year-old male was diagnosed with stage I papillary thyroid cancer (PTC). His TgAbs were negative prior to the diagnosis of thyroid cancer and became positive after thyroidectomy and radioactive iodine administration. A detailed work-up including a whole body scan did not reveal recurrent disease. He had been diagnosed with common variable immune deficiency (CVID) and dermatomyositis at the age of 50 and was started on immunoglobulin (Ig) replacement therapy shortly after diagnosis. His Tg was negative when assessed with liquid chromatography-tandem mass spectrometry (LC-MS/MS). Therefore, elevated TgAb titers were attributed to concomitant SCIg treatment. We also demonstrated that SCIg treatment had TgAb activity that was removed by protein A column treatment. Dilutions of SCIg medication also caused positive IgG serologies for cytomegalovirus and herpes simplex, measles, mumps, rubella, and varicella zoster viruses.

Conclusion: An exogenous source of TgAbs from SCIg led to extensive imaging work-up to assess for PTC recurrence. LC-MS/MS is a conceptually attractive approach to overcome TgAb interference with Tg IMA measurement.