Background: Mixed medullary-follicular carcinoma of the thyroid with a pleomorphic pattern is a rare malignant epithelial tumor characterized by clinical and immunohistochemical features of follicular and parafollicular thyroid cells. Different molecular mechanisms for mixed thyroid tumors have been suggested.
Case Report: We describe a 71-year-old man with a history of malignant melanoma with mixed medullary-follicular thyroid carcinoma. Cytology results of a fine needle aspiration biopsy were suspicious of a thyroid carcinoma. The patient underwent a total thyroidectomy for a solitary thyroid nodule in the right lobe. No lymph node metastases were present. Histopathology and immunohistochemistry revealed a mixed medullary and follicular cell carcinoma that showed characteristic patterns and calcitonin and thyroglobulin positivities in many of the tumor cells. The tumor was not associated with multiple endocrine neoplasia type 2. Detection of RET proto-oncogene point mutations in risk exons 10, 11, 13, 14, 15, and 16 was negative. Two polymorphisms, one in exon 11 G691S (GGT→AGT) and another in exon 15 S904S (TCC→TCG) were detected.
Conclusions: A mixed differentiated thyroid tumor is a diagnostic challenge with fine needle aspiration. Definitive diagnosis remains the domain of histology because of the necessity of topographic information. The origins of this rare tumor entity are unclear. The possible association with the uncommon polymorphism G691S of the RET proto-oncogene is discussed.

Keywords: Follow-Up Studies, Melanoma - pathology, Thyroid Neoplasms - surgery