Which hormone increases blood calcium by stimulating bone resorption?

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

Answer: (D)

Calcium homeostasis is regulated by hormones that coordinate bone remodeling, kidney handling, and intestinal absorption. When calcium is low, parathyroid hormone rises. It increases blood calcium mainly by stimulating bone resorption: PTH boosts signals from osteoblasts that promote osteoclast formation and activity, leading to dissolution of bone and release of calcium into the bloodstream. It also enhances calcium reabsorption in the kidneys and promotes activation of vitamin D, which increases intestinal calcium absorption. Because of these effects, parathyroid hormone is the key hormone that raises blood calcium by bone resorption. Calcium-lowering calcitonin inhibits osteoclasts, while growth hormone and thyroxine influence bone turnover in different ways but do not primarily raise calcium through bone resorption.