Calcitonin is a peptide hormone secreted by parafollicular cells of the thyroid gland, also known as C cells. The main function of calcitonin is to promote calcium absorption by bones, regulate calcium and phosphorus metabolism, and lower blood calcium and phosphorus. Calcitonin promotes bone growth and can also be used as a specific tumor marker for medullary thyroid cancer. Therefore, the increase of serum calcitonin concentration is mainly seen in medullary thyroid cancer and small cell lung cancer, etc. It is mainly used for the diagnosis of medullary thyroid cancer and small cell lung cancer and long-term postoperative tracking and follow-up. In patients with renal insufficiency, serum calcitonin levels can be increased to different degrees. In addition, a mild increase in calcitonin levels can also occur in normal populations such as some children and pregnant women. Decreased serum calcitonin concentrations are seen mainly in patients after thyroidectomy, with a slight decrease in serum calcitonin levels in healthy adult women.