Calcitonin is 32 amino acids polypeptide whose primary function is to inhibit osteoclasts activated bone resorption. In mammals calcitonin is secreted in the thyroid gland by specialized cells called C cells which have embryonic origin in ultimobranchial bodies. C cells are about 0.1% of the mass of thyroid gland. To achieve hypocalcemic effect all 32 amino acids are necessary. From thyroid gland calcitonin is released namely by calcium and magnesium. Calcitonin decreases bone resorption which can be demonstrated in vivo and in vitro. Calcitonin is used to treat osteoporosis and algodystrophic syndrome. In the PROOF study was given salmon calcitonin at a dose 200 units daily for 5 years. In this dose spine bone density increased about 1.5%, however; there was no significant increase of bone density in the hip. Administration of nasal spray of calcitonin in a study PROOF reduced the risk of new fracture by 36% compared to placebo. Intracranially calcitonin acts as a neurotransmitter. According to the predominant view, calcitonin protects the skeleton under conditions of increased calcium demand during growth, pregnancy and lactation. Calcitonin is also important tumor marker of thyroid carcinoma.