Which hormone raises blood calcium levels by acting on bone?

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

Answer: (B)

Blood calcium is kept in balance by hormones that control bone remodeling. When calcium levels fall, parathyroid hormone is released and acts on bone to stimulate osteoblasts to increase RANKL expression, which activates osteoclasts. The osteoclasts break down bone and release calcium into the bloodstream, raising the level. This direct action on bone is the crucial way PTH increases circulating calcium, and it’s complemented by kidney reabsorption of calcium and activation of vitamin D to boost gut absorption.

Calcitonin does the opposite by inhibiting osteoclasts, which lowers blood calcium. Vitamin D mainly increases calcium availability by enhancing intestinal absorption and supporting bone mineralization, with its bone effects not primarily about acutely raising blood calcium via bone resorption. Growth hormone mainly promotes bone formation through osteoblast activity, not by raising blood calcium through bone resorption.