Calcitonin is a 32 amino acid peptide hormone secreted by parafollicular cells (also known as C cells) of the thyroid (or endostyle) in humans and other chordates. It acts to reduce blood calcium (Ca2+), opposing the effects of parathyroid hormone (PTH). 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. It may even be used on biopsy samples from suspicious lesions (e.g., lymph nodes that are swollen) to establish whether they are metastases of the original cancer. Increased levels of calcitonin have also been reported for various other conditions. They include: C-cell hyperplasia, nonthyroidal oat cell carcinoma, nonthyroidal carcinoma and other nonthyroidal malignancies, acute kidney injury and chronic kidney failure, hypercalcemia, hypergastrinemia, and other gastrointestinal disorders, and pulmonary disease. Immunohistochemical staining with anti-calcitonin antibody has proven to be an effective way of demonstrating calcitonin-producing cells in the thyroid. C-cell hyperplasia and medullary thyroid carcinomas stain positive for calcitonin. Studies of calcitonin have resulted in the identification of a wide spectrum of C-cell proliferative abnormalities.