Which parathyroid hormone regulates calcium and phosphorus metabolism and increases blood calcium levels by stimulating osteoclasts?

Which parathyroid hormone regulates calcium and phosphorus metabolism and increases blood calcium levels by stimulating osteoclasts?

• A. Parathyroid hormone
• B. Calcitonin
• C. Melatonin
• D. Thyroid hormone

Answer: (A)

Calcium balance in the blood is controlled by hormones that coordinate bone, kidney, and gut activity. The parathyroid hormone acts as the key regulator that raises blood calcium when it drops. It does this by stimulating osteoclasts to break down bone tissue, releasing calcium (and phosphate) into the bloodstream. It also helps conserve calcium by increasing its reabsorption in the kidneys and by promoting the production of calcitriol, the active vitamin D, which boosts calcium (and phosphate) absorption from the gut. The net result is an elevation of blood calcium. This fits the choice because the parathyroid hormone specifically increases blood calcium levels via bone resorption and other kidney and gut effects. The other options don’t fit this role: calcitonin lowers blood calcium by inhibiting osteoclasts; melatonin isn’t involved in calcium metabolism; thyroid hormone influences overall metabolism and bone turnover but does not primarily regulate calcium levels in the way PTH does.

Calcium balance in the blood is controlled by hormones that coordinate bone, kidney, and gut activity. The parathyroid hormone acts as the key regulator that raises blood calcium when it drops. It does this by stimulating osteoclasts to break down bone tissue, releasing calcium (and phosphate) into the bloodstream. It also helps conserve calcium by increasing its reabsorption in the kidneys and by promoting the production of calcitriol, the active vitamin D, which boosts calcium (and phosphate) absorption from the gut. The net result is an elevation of blood calcium.

This fits the choice because the parathyroid hormone specifically increases blood calcium levels via bone resorption and other kidney and gut effects. The other options don’t fit this role: calcitonin lowers blood calcium by inhibiting osteoclasts; melatonin isn’t involved in calcium metabolism; thyroid hormone influences overall metabolism and bone turnover but does not primarily regulate calcium levels in the way PTH does.