Markers
While the increased serum concentration of calcitonin is not harmful, it is useful as a marker which can be tested in blood.
A second marker, carcinoembryonic antigen (CEA), also produced by medullary thyroid carcinoma, is released into the blood and it is useful as a serum or blood tumor marker. In general, measurement of serum CEA is less sensitive than serum calcitonin for detecting the presence of a tumor, but has less minute to minute variability and is therefore useful as an indicator of tumor mass.
Read more about this topic: Medullary Thyroid Cancer