Which hormone stimulates calcium release from bones?

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

Answer: (A)

Calcium homeostasis is regulated by hormones, and the one that mobilizes calcium from bone is parathyroid hormone. When blood calcium levels drop, the parathyroid glands release this hormone, which stimulates osteoclast activity (via signaling from osteoblasts and RANKL). That causes bone resorption, releasing calcium into the bloodstream. PTH also increases calcium reabsorption in the kidneys and promotes activation of vitamin D, which boosts calcium absorption from the gut. The net effect is an increase in blood calcium. Other hormones like calcitonin decrease bone resorption, vitamin D mainly increases gut absorption, and growth hormone influences bone growth and overall metabolism but does not directly trigger calcium release from bone.