Calcitonin is a hormone that is synthesized in some specialised cells of the thyroid gland. Calcitonin is involved in the bone remodelling process by its role in regulating the blood levels of Ca2+ ions. In this role, calcitonin acts as the counterplayer to parathyroid hormone, PTH. PTH secretion is increased whenever the blood concentration of Ca2+ ions is low or falling, and then PTH activates the osteoclasts (cells responsible for breaking down old bone tissue in the bone renewal process). This increased osteoclast activity increases the Ca2+ concentration in blood by increased breaking down of bone tissue.

Calcitonin has the opposite action trigger and effects. Its secretion is increased by high or increasing Ca2+ concentration in the blood serum. Another activating factor for increased release of calcitonin stems from gastrointestinal hormones such as gastrin. An increased calcitonin concentration mainly inhibits / reduces the activity of osteoclasts in the resorption phase of the bone remodelling, it is thought to have some general effects on osteocytes (common type of cells in mature bone tissue) as well, and also enables increased excretion of Ca2+ (and phosphate) ions in the urine. The main physiological role of calcitonin may thus be summarised as protecting against excessive bone loss from overactive Ca2+ mobilisation. Calcitonin is also known to stimulate vitamin D production in the kidneys. Many of the more subtle aspects of the effects, interactions and roles of calcitonin remain to be clarified. Abnormally high calcitonin levels may indicate some thyroid gland diseases, including malignancies affecting the thyroid cells that produce calcitonin, kidney failure and a range of disorders of the digestive system. One such syndrome produces neuromas (small nerve growths) in the lining of the mouth.

Calcitonin has a role as medication in the treatment of hypercalcaemia (abnormally high Ca2+ concentrations in blood serum) and a number of bone disorders, including Paget’s disease, osteoporosis after menopause (although the use for this condition is nowadays rare, with bisphosphonates and denusomab nowadays much more widely prescribed antiresorptive agents), bone pain from bone metastases. Calcitonin has been mentioned in the literature as a medication for the treatment of some forms chronic osteomyelitis but the evidence is unconvincing.

Calcitonin for use as medication is isolated from salmon. Calcitonin needs to be given by injection into a muscle or under the skin. Calcitonin is a polypetide molecule (a chain of nucleic acid building blocks) that, if taken orally, would be made ineffective because it would be chemically broken down in the acidic stomach environment.