Calcitonin is a polypeptide hormone of 32 amino acids, with a molecular weight of 3454.93 daltons. Calcitonin is produced in humans primarily by the parafollicular (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). It has been found in fish, reptiles, birds, and mammals. Its structure comprises a single alpha helix. Alternative splicing of the gene coding for calcitonin produces a distantly related peptide of 37 amino acids, called calcitonin gene-related peptide (CGRP), beta type.