8426 Recurrence of Papillary Thyroid Carcinoma Despite Undetectable Levels of Thyroglobulin Measured by Both Immunoassay and LCMS

Abstract Disclosure: M.G. Hamilton: None. G. Patel: None. S. Teja Sathi: None. K. Win: None. P. Lekprasert: None. Serum thyroglobulin (Tg) levels are affected by the presence of anti-thyroglobulin antibodies (anti-Tg Abs) during surveillance of differentiated thyroid cancer (DTC). About 20% of patients have positive anti-Tg Abs post-thyroidectomy and laboratory investigations for DTC surveillance may lead to false negative results. Liquid chromatography-tandem mass spectrometry (LCMS) is superior to immunoassay in measuring Tg levels in patients with the presence of anti-Tg Abs. Mass spectrometry is the only currently available method that could eliminate the interference of anti-Tg Abs in the quantification of Tg but is not readily available. We present the case of a patient who had a recurrence of DTC despite persistently undetectable Tg levels using both immunoassay and LCMS methods. The patient is a 49-year-old female with a history of papillary thyroid cancer who underwent total thyroidectomy followed by adjuvant radio-ablation. She presented to our endocrinology office for routine follow up for her DTC. She reported subjective unknown weight loss over four months. Her physical exam was unremarkable with no palpable neck swelling or mass. Vital signs were within normal limits. She continued to have undetectable levels of Tg as measured by immunoassay, stable levels of anti-Tg Abs ranging from 19 IU/mL to 33 IU/mL, and negative findings on her thyroid ultrasound for the first four years post-op. When she presented to the office five years post-thyroidectomy her anti-Tg Ab level had increased to 114 IU/mL and subsequently to 137 IU/mL a few months later despite undetectable Tg levels on both immunoassay and LCMS. A thyroid ultrasound was completed due to rising anti-Tg Abs levels which showed a suspicious one cm mass at the right thyroid bed and suspicious bilateral cervical lymph nodes. Repeated FNA biopsy of the bilateral cervical lymph nodes confirmed follicular epithelium with nuclear inclusions and grooves, compatible with recurrent papillary thyroid carcinoma. This patient had a recurrence of her DTC despite undetectable Tg levels on both immunoassay and LCMS. Tg is an important aspect of surveillance post-treatment for all patients with DTC because its presence is a surrogate for the presence of thyroid tissue and therefore the recurrence of thyroid malignancy. Clinicians must have a high index of suspicion when monitoring a patient with a history of DTC and the presence of anti-Tg Abs because it interferes with the accurate measurement of Tg levels. A study conducted by Azmat et al. demonstrated that measuring Tg levels via LCMS has been found to eliminate the confounding effect of anti-Tg Abs with a sensitivity and specificity of 44.4% and 94.1% respectively. However, our case illustrates that even LCMS is not always reliable in such situations, and increasing levels of anti-Tg Abs should be an individual risk factor for suspecting recurrence of DTC. Presentation: 6/3/2024