SAT-327 Metastatic Papillary Thyroid Carcinoma Unveiled in a Patient with New-Onset Myasthenia Gravis

Abstract Disclosure: A. Razzeto: None. K. Baez: None. J. Mark: None. C.V. Villabona: None. Introduction: Myasthenia gravis (MG) is associated with thymic abnormalities and autoimmune thyroid diseases, most commonly hyperthyroidism. However, emerging evidence suggests a potential link with thyroid cancer. This case highlights a rare presentation of new-onset MG concurrent with metastatic papillary thyroid carcinoma (PTC). Case Presentation: A 58-year-old male with a history of hypertension and hyperlipidemia presented with ptosis, diplopia, fatigue, and progressive lower extremity weakness, numbness and tingling exacerbated at the end of the day. Symptoms, highly suggestive of MG, improved with prednisone and pyridostigmine. Evaluation for causes of neuropathy including heavy metals, vitamin deficiencies (B12, folate, B6), autoimmune disorders (connective tissue, Sjogren's, anti-dsDNA), paraneoplastic panel, acetylcholine receptor, and MuSK antibodies were negative. Thyroid tests showed subclinical hyperthyroidism (TSH 0.26 mIU/L, normal free T4 1.1 ng/dL). EMG confirmed MG, peripheral sensory neuropathy, and carpal tunnel syndrome. Imaging studies revealed a 5 mm pituitary microadenoma and a thyroid nodule, without thymic mass. Subsequent thyroid ultrasound revealed a suspicious 1.3 cm hypoechoic nodule with calcifications and cervical lymphadenopathy. FNA of both confirmed PTC. Thyroglobulin was elevated (69.9 ng/mL), thyroglobulin antibodies negative (<1.0 IU/mL), TSH normalized (0.8 mcIU/mL). Left thyroid lobectomy with neck dissection revealed PTC (pT1bN1) with left recurrent laryngeal nerve invasion and metastases to 14/16 lymph nodes. Completion of right thyroid lobectomy is planned. Conclusion: This case adds to the growing evidence of an association between MG and extrathymic malignancies, including PTC. While reported in a small number of cases, studies suggest that patients with MG have an increased cancer incidence of 1.7%-15.4% vs 2.78% in the general population and a 1.38-fold higher risk of extrathymic neoplasms. Immune dysregulation may play a role in both MG and reduced cancer immunosurveillance. Therefore, clinicians should be vigilant for potential co-occurring cancers, particularly thyroid carcinoma, in patients presenting with new-onset or atypical MG. Presentation: Saturday, July 12, 2025