Fine‐needle aspiration cytology (FNAC) is a preliminary test for the diagnosis of thyroid lesions. We hereby report a rare case of medullary thyroid carcinoma (MTC) co‐existing with Hashimoto's thyroiditis (HT). This case was substantiated with ancillary tests on cytology material to give a novel insight. A 60‐year‐old female presented clinically with diffuse enlargement of the thyroid, and right‐side nodule on ultrasonography. FNAC of the isthmic area showed features of HT, while cytology of right‐side nodule displayed sheets of plasmacytoid cells and frequent scattered large bizarre and pleomorphic cells, lymphoglandular bodies and pale eosinophilic material. The differential diagnosis of MTC with co‐existent HT or high‐grade lymphoma was considered. To establish the diagnosis, serum calcitonin and ancillary studies on aspirated material were carried out. High serum calcitonin (7251 pg/mL), Congophilia on smears, and CD 45−ve; CD56+ve expression of tumor cells on flowcytometric analysis established the diagnosis of MTC over lymphoma. This diagnosis was further confirmed upon histopathology. Ancillary studies on aspirated material established the diagnosis of MTC and excluded the diagnosis of lymphoma. Establishing the correct diagnosis was cardinal in such a scenario as these diseases have extremely diverse management.

No abstract