Abstract
Papillary thyroid and medullary thyroid cancers are two distinct types of thyroid neoplasms. Co-occurrence of these cancers is rare, especially in mixed tumours with lymph node metastases. A 66-year-old man presented with a thyroid tumour. Thyroid ultrasonography revealed three separate nodules in the thyroid, suspected to be associated with lymph node metastasis. Although preoperative thyroid function was normal, calcitonin and carcinoembryonic antigen levels were elevated. The patient underwent a total cervical thyroidectomy with bilateral radical dissection. Histological and immunohistochemical analyses identified mixed medullary and papillary thyroid carcinoma (MMPTC) in the nodules in the left lobe of the thyroid and the isthmus. Mixed metastatic spread was observed in several lymph nodes from the neck dissection specimen. Accurate diagnosis of the rare co-occurrences of papillary and medullary thyroid carcinomas is crucial. TSH suppression can be effective for treating papillary thyroid carcinoma, whereas radical surgery is the preferred treatment for medullary thyroid carcinoma. Identifying lymph node metastasis before surgery is a key surgical strategy.