Calcitonin (salmon) is a 32-amino acid polypeptide hormone that is produced in humans primarily by the parafollicular cells (also known as C-cells) of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium (Ca2+), opposing the effects of parathyroid hormone (PTH). The calcitonin receptor has been cloned and shown to be a member of the seven-transmembrane, G protein-coupled receptor family.