Abnormal Antithyroglobulin Antibody in a Woman With Papillary Thyroid Carcinoma Receiving Intravenous Immunoglobulin...

Thyroglobulin and antithyroglobulin antibodies are used in the surveillance of differentiated thyroid carcinomas. In patients with differentiated thyroid carcinoma who require intravenous immunoglobulin for autoimmune or malignant conditions, surveillance of antithyroglobulin antibodies is challenging. We present a case of a patient with papillary thyroid carcinoma who showed fluctuating antithyroglobulin antibody levels while receiving intravenous immunoglobulin.

A 41-year-old woman underwent total thyroidectomy, central and left modified neck dissection, and radioactive iodine ablation for multifocal papillary thyroid carcinoma with features of oncocytic and warthin-like variant and metastasis to 1 of 28 left lateral and 1 of 10 central lymph nodes. Ten years later, she contracted COVID-19 and developed COVID-19-associated encephalopathy for which she received intravenous immunoglobulin for 7 months. Five months after intravenous immunoglobulin initiation, surveillance thyroglobulin was <0.8 ng/mL (liquid chromatography with mass spectrometry, <0.4 ng/mL) and antithyroglobulin antibodies increased from undetectable to 97 IU/mL (chemiluminescent immunoassay, ≤1 IU/mL). After the final intravenous immunoglobulin dose, antithyroglobulin antibody levels progressively decreased, reaching 1 IU/mL (chemiluminescent immunoassay, ≤1 IU/mL) 6 months posttreatment. Neck ultrasounds showed no evidence of disease recurrence.

Intravenous immunoglobulin can affect monitoring of patients with differentiated thyroid carcinomas due to passive transfer of pooled donor antibodies, including antithyroglobulin antibodies. Newly detectable antithyroglobulin antibodies in this patient were likely due to measured donor antithyroglobulin antibodies via intravenous immunoglobulin rather than cancer recurrence.

In patients with differentiated thyroid carcinomas receiving intravenous immunoglobulin, the presence of donor antithyroglobulin antibodies should be considered when antithyroglobulin antibodies are newly detectable.