SUN-341 Double trouble: A rare case of coexisting metastatic medullary and papillary thyroid carcinoma

Abstract Disclosure: K. Sheikh: None. The authors describe the case of a 48 year old male with type 2 diabetes mellitus and hypertension who presented to his primary care physician with complaints of left ear pain and diminished hearing. An MRI of the brain was performed, revealing chronic ischemic changes. A CT neck revealed a 1.1 cm hypodense nodule in the right thyroid lobe, a 2.0 x 2.5 cm heterogeneous partially necrotic nodule posterior to the right thyroid lobe, and a 3.1 x 2.0 cm heterogeneous partially necrotic right level IIIB lymphadenopathy. Fine needle aspiration (FNA) and core needle biopsy of the partially necrotic lymph node were completed and pathology demonstrated findings of both medullary thyroid cancer (MTC) and papillary thyroid cancer (PTC). Lab values showed CEA 34.4 ng/mL (normal< 3.9 ng/mL) , calcitonin 1120 pg/mL(normal <8.4 pg/mL), parathyroid hormone (PTH) 35 pg/mL (15-65 pg/mL), and TSH 2.23 uIU (normal 0.450-4.50 uIU. The patient underwent a total thyroidectomy, bilateral central neck dissection, right modified radical neck dissection, and decompressive neurolysis. The final pathology confirmed a distinct 3mm MTC tumor and a 1.0 mm PTC tumor in the right thyroid lobe. MTC/PTC is a rare presentation of thyroid malignancies in which distinct MTC and PTC tumors are present in the same individual. This case demonstrates the importance of comprehensive biochemical and histological analysis of thyroid malignancies to ensure that mixed thyroid cases are properly diagnosed and treated. Presentation: Sunday, July 13, 2025