What is the role of calcitonin in calcium homeostasis?

• A. Increases urinary calcium excretion
• B. Lowers serum calcium by inhibiting bone resorption
• C. Decreases intestinal calcium absorption
• D. Elevates PTH secretion

Answer: (B)

When calcium levels are high, the body uses calcitonin to bring them down. Calcitonin is released from the thyroid's C cells and acts mainly on bone: it inhibits osteoclasts, the cells that break down bone and release calcium. With osteoclast activity suppressed, less calcium is released from bone into the bloodstream, so serum calcium falls. This is the primary way calcitonin helps maintain calcium balance.

It doesn't significantly decrease intestinal calcium absorption; that process is mainly influenced by vitamin D. It also doesn’t promote PTH secretion—PTH is the hormone that raises calcium when it's low, so calcitonin and PTH have opposite effects. While there may be minor renal effects, the key, exam-worthy action is the inhibition of bone resorption to lower serum calcium.