This case report presents a rare coexistence of multiple medullary thyroid carcinoma (MTC) and papillary thyroid carcinoma (PTC) in a 43-year-old woman with a 6-month history of a thyroid mass. Physical examination revealed a firm, irregular isthmus nodule without signs of inflammation. Laboratory tests showed elevated serum calcitonin, carcinoembryonic antigen and gastrin-releasing peptide precursor. Ultrasonography revealed multiple hypoechoic nodules in both thyroid lobes and the isthmus, classified as TI-RADS 4a (right and left lobes), 4b (right lobe), and 3 (isthmus). Postoperative pathology confirmed PTC in the left lobe and MTC in the right lobe and isthmus, both with lymph node metastases. This case underscores the diagnostic challenges of synchronous thyroid malignancies and highlights the key role of high-resolution ultrasound in differentiating pathological subtypes. Comprehensive imaging assessment, combined with biochemical markers and histopathology, is essential for accurate diagnosis and tailored treatment.