Calcitonin is a hormone produced by the thyroid gland that lowers the levels of calcium and phosphate in the blood (when calcium levels rise above normal) and promotes the formation of bone. The major source of calcitonin is from the parafollicular or C cells in the thyroid gland, but it is also synthesized in a wide variety of other tissues, including the lung and intestinal tract.
Resources:
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/thyroid/calcitonin.html