Which hormone increases blood calcium by stimulating osteoclast activity?

• A. Parathyroid hormone
• B. Calcitonin
• C. Vitamin D
• D. Thyroxine

Answer: (A)

Blood calcium is kept in balance by hormones that control bone breakdown, calcium absorption, and kidney reabsorption. The hormone that raises blood calcium by directly promoting osteoclast activity is parathyroid hormone. When calcium levels fall, the parathyroid glands release PTH. It acts on osteoblasts to increase the expression of RANKL, which activates osteoclasts to resorb bone and release calcium into the bloodstream. PTH also signals the kidneys to reabsorb more calcium and to produce active vitamin D, which increases calcium absorption from the gut, all contributing to higher serum calcium.

Calcitonin lowers blood calcium by inhibiting osteoclasts, so it acts in the opposite direction. Vitamin D helps increase calcium availability by boosting gut absorption and supporting bone mineralization, but its primary effect isn’t stimulating osteoclasts. Thyroxine can increase bone turnover when present in excess, but it isn’t the hormone whose main action is triggering osteoclast-mediated calcium release.