Serum calcitonin negative mixed medullary-follicular carcinoma initially diagnosed as medullary thyroid carcinoma by fine-needle aspiration cytology: A case report and review of the literatures.

Medullary thyroid carcinoma (MTC) is potentially lethal. A prompt and accurate diagnosis is the prerequisite for the treatment of MTC. Fine-needle aspiration (FNA) is a reliable diagnostic tool in the assessment of thyroid nodules. However, cytologic assessment of MTC based on FNA has several drawbacks due to morphological variants. We present a case of MTC diagnosed through FNA cytology, which was eventually histologically confirmed as a mixed medullary-follicular carcinoma with negative serum calcitonin expression. Hence, diagnosis of MTC based on FNA should be applied with caution. Ultrasound characteristics of suspicious thyroid nodules are recommended to be evaluated by FNA. However, calcitonin levels should be measured in both the FNA washout fluid and serum when features of MTC are presented or cytology result is inconclusive. If adequate FNA sample is available, a supplementary immunocytochemical staining of markers such as calcitonin, chromogranin, carcinoembryonic antigen, and thyroglobulin is helpful for a correct diagnosis of MTC.