The discovery of calcitonin established the presence of a new regulatory system for calcium homeostasis. Calcitonin, a hormone isolated from the human thyroid gland, is a single-chain, 32 amino acid polypeptide with a molecular weight of 3600.
Calcitonin is produced by the mitochondria-rich parafollicular or C-cells of the thyroid and is secreted in response to elevations in plasma calcium concentration and in response to a numb her of gut hormones, including gastrin and glucoagon.
Other tissue sources of calcitonin include the pituitary, a variety of neuroendocrine cells, and carcinomatus lesions such as medullary thyroid carcinoma and small cell lung cancer.
Bone and kidney are the two primary targets of calcitonin and both are important for the activity of calcitonin to acutely decrease circulating levels of calcium.
In bone, calcitonin acts directly on osteoblasts, reducing calcium release into the plasma through inhibition of bone resorption, whereas in the kidneys calcitonin reduces reabsorption of both calcium and phosphorus, producing a modest increase in excretion of both ions. The former may contribute to the hypocalcaemic effect of pharmacologic effect of pharmacologic doses of calcitonin.
Hormone of calcitonin
