Calcitonin (also known as thyrocalcitonin) is a 32-amino acid linear polypeptide hormone. It acts to reduce blood calcium (Ca2+), opposing the effects of parathyroid hormone (PTH). Calcitonin is a hormone known to participate in calcium and phosphorus metabolism. In mammals, the major source of calcitonin is from the parafollicular or C cells in the thyroid gland, but it is also synthesized in a wide variety of other tissues, including the lung and intestinal tract. There are 2 numbers of copies of the gene in the human genome on chromosome 11. It may be used diagnostically as a tumor marker for medullary thyroid cancer, in which high calcitonin levels may be present and elevated levels after surgery may indicate recurrence. Increased levels of calcitonin have also been reported for various other conditions. They include: C-cell hyperplasia, Nonthyroidal oat cell carcinoma, Nonthyroidal small cell carcinoma and other nonthyroidal malignancies, acute and chronic renal failure, hypercalcemia, hypergastrinemia and other gastrointestinal disorders, and pulmonary disease.