High Thyroglobulin Antibody Following Intravenous Immunoglobulin Therapy in Athyreotic Differentiated Thyroid Cancer Patients

American Thyroid Association guidelines recommend to follow athyreotic differentiated thyroid cancer patients with measurement of serum thyroglobulin and thyroglobulin antibody as tumor markers. The guidelines recommend that rising thyroglobulin or thyroglobulin antibody should prompt additional investigations and potentially additional therapies. Two patients with differentiated thyroid cancer who also received intravenous immunoglobulin are presented. Their cancer history, serial thyroglobulin and thyroglobulin antibody measurements and imaging findings relative to the time course of intravenous immunoglobulin treatment are illustrated. Acute rise in thyroglobulin antibody led to further imaging which did not show cancer progression. Additional history documented an intravenous immunoglobulin treatment exposure had occurred within the past one to two months before the increased thyroglobulin antibody measurement. Follow-up serial thyroglobulin antibody levels declined over time after the intravenous immunoglobulin exposure. Intravenous immunoglobulin, which is a pooled human serum product, contains thyroglobulin antibody. Commercially available thyroglobulin antibody assays may detect the thyroglobulin antibody contained within the administered intravenous immunoglobulin, leading to alarm and further imaging to exclude progressive malignancy. Thyroglobulin antibody rise and fall can be demonstrated in relationship to intravenous immunoglobulin time of administration. Thyroglobulin antibody is higher at time-points sooner than at later time-points following intravenous immunoglobulin treatments. Intravenous immunoglobulin may be a benign source of transiently high thyroglobulin antibody measured in the follow-up of differentiated thyroid cancer patients. Repeat thyroglobulin and thyroglobulin antibody testing one to two months following a higher level in a patient treated with intravenous immunoglobulin may avoid unnecessary imaging to look for progressive malignancy.