Calcitonin is a hormone that helps regulate calcium levels in the blood. It works by:

* Decreasing blood calcium levels: It inhibits the activity of osteoclasts, the cells that break down bone, and stimulates the activity of osteoblasts, the cells that build bone. This reduces the amount of calcium released from bones into the bloodstream.

* Increasing calcium excretion: It promotes the excretion of calcium in the urine, further lowering blood calcium levels.

Overall, calcitonin acts as a counter-regulatory hormone to parathyroid hormone, which raises blood calcium levels.