Calcitonin is a hormone produced by the thyroid gland that helps regulate calcium levels in the body. It lowers blood calcium levels by inhibiting osteoclast activity, which decreases bone resorption, and promoting calcium excretion by the kidneys. This action helps to balance the effects of parathyroid hormone, which increases blood calcium levels. Calcitonin is also used therapeutically to treat conditions like osteoporosis and Paget's disease.