CEACAM5 and medullary thyroid gland carcinoma: Biochemically, serum calcitonin, secreted specifically by parafollicular C cells, is a highly sensitive biomarker for the diagnosis and postoperative monitoring of MTC. CEA is a nonspecific tumor marker used as a complementary tumor marker, particularly useful in monitoring disease progression and recurrence when interpreted alongside calcitonin [1, 2, 4].