Which hormone increases blood calcium levels?

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

Answer: (D)

Calcium homeostasis is tightly regulated by parathyroid hormone, which increases blood calcium when levels fall. It acts on bone to promote osteoclast-mediated resorption, on the kidneys to increase calcium reabsorption and to activate vitamin D, and on the gut indirectly by raising calcitriol that boosts intestinal calcium absorption. The net effect is higher serum calcium. Calcitonin lowers calcium by inhibiting osteoclasts, so it does not raise calcium. Adrenaline and melatonin are not primary regulators of calcium balance.