Medullary thyroid carcinoma (MTC) is the differentiated thyroid cancer arising from the neoplastic clonal proliferation of parafollicular C-cells within thyroid parenchyma. It is widely believed to be neuroectodermal in origin. We highlight a rare case scenario, wherein the pathological biology of tumour changed to a mesodermal plasmacytoma in the course of followup. Here we discuss this uniquely rare biological phenomenon of MTC exhibiting epithelial- mesenchymal transformation (EMT) at follow-up, drastically changing the course of management.