Introduction. Mixed medullary-follicular thyroid carcinoma is an uncommon tumor that consists of both follicular and parafollicular cells. Case. We report a 43-year-old woman with a palpable mass in the right side of the neck. Fine needle aspiration suggested a diagnosis of high grade anaplastic carcinoma that has been associated with papillary features. Total thyroidectomy was done in which histopathological examination showed diagnosis of medullary carcinoma. Immunohistochemical staining was positive for chromogranin, calcitonin, and thyroglobulin in tumoral cells. Conclusion. Mixed medullary-follicular thyroid carcinoma is a rare tumor. Diagnosis of these tumors with fine needle aspiration is very difficult and may lead to misdiagnosis. It is necessary to correlate the cytological finding with serum calcitonin and thyroglobulin. Also immunostaining for calcitonin and thyroglobulin confirms diagnosis. 1. Introduction Medullary thyroid carcinoma (MTC) is a neuroendocrine tumor that originates from parafollicular or c-cells of the thyroid gland . MTC accounts for 5–10% of all thyroid carcinomas. A characteristic feature of this tumor is production of calcitonin. Most MTCs (80%) are sporadic and others (20%) are familial [2, 3]. Pfaltz et al. in 1959 reported that MTC is a tumor with solid-nonfollicular pattern, and it is different from other thyroid carcinomas clinically and pathologically . Also it was noted that histological appearance of MTC may be atypical, and follicular structures can be seen in it. Also it was shown that these tumors have positive immunoreactivity for calcitonin and thyroglobulin. Subsequently such tumors were named mixed medullary-follicular carcinoma [4, 5]. Mixed medullary-follicular carcinoma is a rare tumor of the thyroid. Less than 40 cases have been reported in the literature [4–8]. The cellular origin of the mixed medullary-follicular tumors is unknown. Here we report a particular presentation of a mixed medullary-follicular carcinoma in which fine needle aspiration showed anaplastic carcinoma and mixed medullary-follicular thyroid carcinoma confirmed after thyroidectomy. 2. Case A 43-year-old woman presented with a palpable mass in the right side of the neck, increasing in size over several months. Physical examination revealed a 4？cm ？×？ 4？cm, well-defined nontender nodule in right thyroidal lobe. There were no other physical abnormalities. The patient didnot smoke. Her family history was negative for thyroid and other endocrine tumors. The patient had no history of neck or whole body radiation. The plasma levels
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