Which hormone increases blood calcium and promotes bone resorption?

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

Answer: (B)

Calcium homeostasis relies on a hormone that raises blood calcium when levels are low by triggering bone resorption, increasing kidney calcium reabsorption, and promoting intestinal calcium absorption. The parathyroid hormone does exactly that: it stimulates osteoblasts to express signals that activate osteoclasts, leading to bone breakdown and release of calcium into the blood. It also enhances calcium reabsorption in the kidneys and stimulates production of the active form of vitamin D, which increases intestinal calcium absorption. The net effect is an increase in serum calcium and a move toward restoring balance.

Calcitonin lowers blood calcium by inhibiting osteoclasts, opposite what’s described here. Vitamin D aids calcium absorption from the gut but doesn’t primarily drive bone resorption. Growth hormone influences bone growth and remodeling but isn’t the main regulator that increases blood calcium via bone breakdown.