Calcitonin is a hormone secreted by the thyroid gland that reduces the concentration of blood calcium level when it has risen to an above normal level. This polypeptide hormone is made up of 32 amino acids and is produced mainly by the parafollicular cells (also known as C cells) of the thyroid gland. In fish, birds and other non-mammalian animals, calcitonin is produced by cells of the ultimobranchial body.
Calcitonin opposes the effects of parathyroid hormone (PTH), which acts to increase the blood calcium level. Calcitonin lowers blood calcium levels by suppressing osteoclast activity in the bones and increasing the amount of calcium excreted in the urine. The hormone also prevents the absorption of calcium from the intestine, as well as increasing the calcium absorbed by the kidney. However, the production and increased activity of calcitonin is only short-lived and many patients with chronic hypercalcemia (raised blood calcium) do not have high blood calcitonin levels. A high calcitonin concentration can be an indicator of medullary thyroid carcinoma, a cancer of the parafollicular cells which leads to the secretion of large amounts of calcitonin.
Calcitonin was first isolated and purified in the laboratory in 1962 by Copp and Cheney. The molecule is formed through proteolytic cleavage of a larger prepropeptide, which is coded for by the CALC1 gene. This gene belongs to the superfamily of related protein hormone precursors that includes calcitonin gene-related peptide, islet amyloid precursor protein, and the precursor of adrenomedullin.
Calcitonin is manufactured in the laboratory and used as a medicine to treat or prevent conditions that lead to loss of calcium from the bones. For example, it is a useful treatment in osteoporosis – a condition that causes brittle and easily fractured bones. Calcitonin is also useful in the treatment of other bone diseases such as Paget’s disease, where the renewal and repair cycle in bone is disrupted, leading to large and malformed bones. Another use of calcitonin is in advanced cancer that has spread and started to involve the bones. Patients who have been immobilized for long periods of time due to major fracture caused by osteoporosis, for example, tend to lose bone minerals and calcitonin may also benefit these patients by preventing acute bone loss.
Calcitonin-containing medicines are usually administered as solutions that can be injected but for the past couple of decades, they have also been available in the form of nasal sprays.