Calcitonin, also called thyrocalcitonin, a protein hormone synthesized and secreted in humans and other mammals primarily by parafollicular cells (C cell) in the thyroid gland. At first it was believed that this hypocalcemic agent was produced by the parathyroid glands. Shortly thereafter, however, it was demonstrated that calcitonin was produced specifically within the parafollicular cells (C cells) of the thyroid gland giving rise to the name 'thyrocalcitonin'. More recent studies have shown calcitonin is also produced by the brain, breast, placenta and other neuroendocrine cells.

Calcitonin is a 32 amino acid linear polypeptide hormone discovered in 1962 as a hormone postulated to regulate the 'normal level' or 'tone" of calcium in the circulation. It is formed by the proteolytic cleavage of a larger pre-propeptide, which is a product of the CALC1 gene. And it contains s single disulfide bond, which causes the amino terminus to assume the shape of a ring. Alternative splicing of calcitonin pre-mRNA can yield a mRNA encoding calcitonin gene-related peptide; that peptide appears to function in the nervous and vascular systems.