What is the role of calcitonin?

• A. To decrease calcium levels in the blood.
• B. To increase calcium levels in the blood.
• C. To regulate glucose metabolism.
• D. To stimulate thyroid hormone production.

Answer: (A)

Calcitonin lowers blood calcium levels by limiting bone breakdown. It is produced by the thyroid’s parafollicular (C) cells and is released when calcium is high. Its main action is to inhibit osteoclasts, the cells that dissolve bone and release calcium into the blood. With osteoclast activity reduced, calcium moves into bone, and there’s a modest increase in calcium excretion by the kidneys. In humans, this effect helps counteract high calcium levels, though it’s not the primary regulator—parathyroid hormone and vitamin D play bigger roles.

So the statement that calcitonin decreases calcium levels in the blood fits its function. It does not raise calcium, it does not regulate glucose metabolism, and it does not stimulate thyroid hormone production.