MON-352 New Appearance of Antithyroglobulin Antibodies (Tgabs) as a Marker for Recurrent Papillary Thyroid Carcinoma (Ptc) After Thyroidectomy and Radioiodine Ablation (Raia)

Abstract Disclosure: Y.R. Dadi: None. M.G. Jakoby: None. V. Williams: None. Serum TgAbs are detectable prior to surgery in 10-25% of patients with differentiated thyroid carcinomas (DTC). In addition to interfering with accurate measurements of thyroglobulin (Tg) in immunoassays, rising TgAbs titers may also indicate persistent or recurrent thyroid carcinoma. We present a case of recurrent PTC indicated by new appearance of a rising TgAbs titer many years after initial management. A 74-year-old female was diagnosed with classic variant PTC (T3N0M0) in 2010 at age 60 and managed by total thyroidectomy, central lymph node dissection, and RAIA. RAIA was repeated in 2012 due to persistently detectable Tg level and uptake in the thyroid bed on diagnostic radioiodine whole body scan (WBS). Tg levels subsequently became undetectable, and repeat diagnostic radioiodine WBS in 2014 and 2017 did not show abnormal radioiodine uptake. No thyroid bed masses or abnormal lymph nodes were observed on annual diagnostic neck ultrasonography 2012-2023. However, TgAbs titer, which was undetectable at diagnosis and throughout postsurgical surveillance, became detectable October 2022 and steadily increased to a maximum of 8.6 IU/mL (0-4) January 2024. Neither diagnostic neck ultrasonography January 2023 nor radioiodine WBS February 2023 showed evidence of recurrent thyroid carcinoma, but two enlarged left level III lymph nodes were observed on neck ultrasonography January 2024. No radioiodine uptake was detectable on repeat WBS, but the lymph nodes showed significant uptake of fluorodeoxyglucose (FDG) on PET imaging. Tg levels remained undetectable. Fine needle biopsy of the largest lymph node yielded PTC on cytology. Neck dissection was performed, and only the two PET positive lymph nodes showed metastatic PTC. Following surgery, the patient’s TgAbs titer has steadily decreased.This case is of interest because rising TgAbs titer was the initial marker for recurrence of PTC more than 10 y after apparently successful treatment in a patient with no detectable TgAbs prior to surgery and RAIA. Eradication of all thyroid tissue and its antigenic components by surgery and RAIA has been demonstrated to result in resolution of detectable antibodies to all major thyroid antigens; however, studies of TgAbs as a marker for DTC recurrence have failed to consistently show that presence of antibodies or a rising antibody titer has prognostic significance. Current American Thyroid Association guidelines for DTC recommend FDG-PET if TgAbs titer is > 10 IU/L and WBS is negative, though this case indicates that steadily rising TgAbs titer regardless of absolute TgAbs level may be another indication for PET imaging. Detectable TgAbs and rising titer in a patient without detectable TgAbs prior to treatment and managed by both surgery and RAIA is a concerning marker for persistence or recurrence of DTC and should be evaluated with the same urgency as a rising Tg level. Presentation: Monday, July 14, 2025