Calcitonin is commonly used clinically in units of pg/ml and ng/L. According to the unit conversion of 1ng=1000pg, pg/ml is equivalent to ng/L. Calcitonin is a hormone produced and released by the thyroid gland that regulates calcium levels in the blood and antagonizes parathyroid hormone, which increases blood calcium levels. Normal serum concentrations of calcitonin range from 0 to 8.4 ng/L in men and from 0 to 5.0 ng/L in women. If calcitonin increases significantly, it suggests the possibility of tumor, such as medullary thyroid carcinoma, the tumor originates from parafollicular cells of the thyroid gland, the patient’s serum calcitonin level is often tens to hundreds of times higher than the normal, and the monitoring of calcitonin changes is helpful for the judgment of the disease and the evaluation of the postoperative period. If the calcitonin level is too low, it is common in patients with surgical thyroidectomy and severe hyperthyroidism. When there is a significant abnormality in the calcitonin index, it is necessary to actively consult the doctor and follow the doctor’s instructions for rational treatment.