Calcitonin-negative medullary thyroid carcinoma combined with multifocal papillary thyroid carcinoma: a case report and literature review

Calcitonin-negative medullary thyroid cancer (CTNMTC) is a rare primary thyroid neuroendocrine carcinoma characterised by medullary thyroid carcinoma (MTC) morphology but without elevated serum calcitonin levels. However, the concurrent occurrence of CTNMTC and papillary thyroid carcinoma (PTC) is even rarer. We report a case of a 68-year-old male patient with normal serum calcitonin levels preoperatively, cytological and histopathological findings consistent with MTC morphology, and no detection of calcitonin by immunohistochemistry but positive reactions for thyroid transcription factor-1 (TTF-1) and synapsin (Syn). Additionally, histopathology indicated multifocal PTC. The diagnosis was calcitonin “double-negative” MTC combined with multifocal PTC, which, to our knowledge, is the first such case reported in the literature. It is prone to misdiagnosis due to its distinctive serologic and immunohistochemical manifestations. We also reviewed the relevant literature to provide a basis for the diagnosis and postoperative follow-up of such patients.