Serum calcitonin is a hormone secreted by thyroid follicular cells that, when elevated for some reason, increases the excretion of calcium by the kidneys and lowers blood calcium concentrations. Endemic goiter is a type of goiter. In the early stages, there are no obvious clinical symptoms. The thyroid gland is mildly to moderately diffusely enlarged, soft and painless. Very few significantly enlarged cases may show symptoms of pressure, such as dyspnea, dysphagia, hoarseness, and irritating cough. A retrosternal goiter may have symptoms of esophageal or superior vena cava compression. So, what are the causes of elevated serum calcitonin levels? The following is a brief introduction: When medullary thyroid carcinoma is present, the patient’s serum calcitonin concentration increases and blood calcium concentration decreases. The main cause of medullary thyroid carcinoma is RET proto-oncogene mutation, while about 95% of hereditary MTC and 70% of sporadic MTC are caused by RET mutation of proto-oncogene located at 10q11.2. Serum calcitonin levels increase in pregnant women and children due to bone growth, and decrease in women after menopause. Serum calcitonin levels may be significantly increased in renal insufficiency such as acute and chronic glomerulonephritis, pyelonephritis, renal tuberculosis, acute tubular degeneration and necrosis caused by chemical and biological toxicants, renal tumors and congenital kidney diseases. Lung cancer or tuberculosis, pneumoconiosis, and silicosis can coexist with lung cancer, and the incidence of cancer in these cases is higher than normal. In addition, chronic inflammation of the lung bronchi and fibrotic scarring lesions of the lung can develop into lung cancer. Serum calcitonin levels may be significantly elevated in these patients.