SUN-625 A False Alarm: Elevated Thyroglobulin Levels in a Patient Receiving Intravenous Immunoglobulin

Abstract

Introduction: Thyroglobulin (Tg) is used as a surveillance marker in those with a history of differentiated thyroid cancer. Thyroglobulin antibodies (TgAb) interfere with Tg measurement. Here we present a case with transiently elevated TgAb and Tg levels in a patient receiving intravenous immunoglobulin (IVIG). Case: A 36-year-old female with a history of SLE transitioned her medical care to our facility. She had previously undergone total thyroidectomy following discovery and fine needle aspiration of a 1.7 cm thyroid nodule. Pathology at that time demonstrated a follicular adenoma and micro papillary thyroid carcinoma for which she underwent radioactive thyroid ablation according to practice prior to the 2015 American Thyroid Association guidelines. She remained in remission for 7 years. During this time, surveillance Tg and TgAb were negative using an immunometric (IMA) assay. Her exam during the visit revealed a palpable left cervical lymph node. Tg and TgAb were drawn and returned detectable via radioimmunoassay (RIA). Repeat levels were similar. Although unlikely, this raised concern for cancer recurrence. Neck ultrasound obtained showed a benign-appearing lymph node and no suspicious lesion in the surgical bed. Detailed history revealed that the patient had recently begun monthly IVIG infusions for her SLE. The most recent infusion being 4 days prior to TgAb and Tg levels. Given the reassuring neck ultrasound and the fact that patient’s TgAb and Tg had been undetectable previously, the elevated Tg was thought to be secondary to transmission of TgAb via IVIG. The patient was subsequently followed clinically with serial TgAb and Tg. IVIG therapy was discontinued, repeat Tg levels were undetectable and remained so 4 months later confirming the diagnosis (Table 1). Table 1:Tg=ng/ml TgAb=IU/ml08/10/18 Tg=<0.1 TgAb=<20.0 via IMA 05/31/18 Tg=<0.1 TgAb=<20.0 via IMA 04/11/18Tg=3.70 TgAb=7.0 via RIA04/04/18Tg=4.90 TgAb=7.2 via RIA(Last dose IVIG 4 days prior)12/07/12 Tg=<0.1 TgAb=<20.0 via IMA 10/19/12 Tg=<0.2 TgAb=<20.0 via IMA Discussion: IVIG is known to be associated with passive transfer of antibodies. Elevated TgAb levels have been reported with subcutaneous immunoglobulins. TgAb cause inference when IMA is utilized and can lead to underestimation of Tg levels. In the setting of these Ab, RIA has less interference but is known to cause false elevation in Tg levels. Elevated Tg levels can be problematic for thyroid cancer surveillance. However, as shown in our patient’s case this elevation is only transient. We suspect that there was passive transmission of TgAb in our patient via IVIG leading to detectable Tg levels using RIA. In such patients, IMA could still be used if drawn before IVIG. We recommend reassuring patients in these situations and obtaining serial TgAb and Tg. Especially checking levels a few days prior to the IVIG infusion to help avoid unnecessary work up.