Calcitonin's a 32-amino acid linear polypeptide hormone that's produced in humans primarily by the parafollicular cells (also known as C-cells) of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium (Ca2+), opposing the effects of parathyroid hormone (PTH). It's been found in fish, reptiles, birds, and mammals. Its importance in humans hasn't been as well established as its importance in other animals, as its function's usually not significant to r… (More on Calcitonin)