SAT546 Papillary Thyroid Cancer Surveillance: Thyroglobulin Levels Affected by HAMA

Disclosure: J. Sanchez Perez: None. M. Obeid: None. M. Fariduddin: None. D.J. Toft: None. Introduction: Thyroglobulin (TG) measurement is a major means of detecting recurrence of previously treated differentiated thyroid cancers. Human-Anti Mouse Antibodies (HAMA) can interfere with lab measurements of TG levels and affect clinical decisions and treatment for these cancers.Clinical Case: A 65-year-old female was diagnosed with papillary thyroid cancer (PTC). Surgical pathology following total thyroidectomy demonstrated a tumor of 3.5 cm in its greatest dimension, unifocal, without invasion of locoregional tissue classified as Stage I (pT2N0M0) per AJCC 8th edition. Radioiodine whole body scan (WBS) demonstrated iodine uptake in the neck and 126.5 mCi of I-131 was given. In surveillance 5 years later, TSH was found to be 0.17 mcIU/ml (ref range: 0.35 - 4.00) TG 64.7 ng/ml (ref range: 1.3 - 31.8 ng/mL), and anti-Tg antibodies were undetectable. Neck US consistently showed nonspecific findings of stable small nodes with normal morphology in both supraclavicular regions. Given elevated TG levels, 250 mCi of I-131 was given. Follow-up TG levels ranged from 27.8 to 36.4 ng/ml. A repeat WBS did not reveal any uptake, but as TG remained elevated, I-131 (238 mCi) was again given. Subsequent TG throughout the following 15 years ranged from 2.8 - 58.6 ng/ml. Repeat Neck US only showed benign-appearing lymph nodes. WBS did not have any focal activity to suggest recurrent disease and PET/CT did not show any abnormal FDG uptake. The persistently high TG levels without clinical evidence of tumor recurrence raised the suspicion of lab interference. The serum sample was treated with a blocking reagent that contained mouse immunoglobulin after which the TG level decreased to 2.7 ng/ml compared to untreated value of 20.5 ng/ml.Conclusion: HAMA are human antibodies against mouse antibodies which are believed to develop in humans from direct contact with rodents, including mice, and from the recent increase in the use of mouse monoclonal antibodies for diagnostic and therapeutic purposes. These antibodies implicate an unexpected source of interference in immunoassays, as seen in our patient, which can confound laboratory results and create a clinical dilemma. This patient with PTC had a complete radiological response after surgery, but an incomplete biochemical response which caused the patient to be given a total of 614 mCi of I-131. This case highlights the importance of recognizing the possible presence of HAMA when analyzing TG levels, particularly when the TG levels are used for treatment decisions. Presentation Date: Saturday, June 17, 2023