This review synthesizes actionable evidence on calcitonin (Ctn) and carcinoembryonic antigen (CEA) baseline values and kinetics, universal germline RET testing, and tumor somatic profiling in advanced or progressive disease, and highlights desmoplastic stromal reaction (DSR) as an underused postoperative risk modifier in sporadic MTC. The gene discussed is CEACAM5; the disease is medullary thyroid gland carcinoma.