Levels of carcinoembryonic antigen (CEA)can also be elevated in MTC and used as a tumor marker in surveillance.16 As MTCs arise from C-cells which do not take up iodine, it is not classifiedas a differentiated form of thyroid cancer and is not responsive to radioactiveiodine adjuvant treatment (RAI). Here, CEACAM5 is linked to medullary thyroid gland carcinoma.