Which hormone is primarily responsible for increasing blood calcium?

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

Answer: (C)

Parathyroid hormone is the main hormone that raises blood calcium. When calcium levels drop, the parathyroid glands release PTH, which increases calcium in three key ways: it stimulates bone-resorbing cells to release calcium into the bloodstream, it enhances calcium reabsorption in the kidneys to reduce urinary loss, and it activates vitamin D in the kidneys to form calcitriol, which boosts intestinal calcium absorption. The net result is an increase in serum calcium toward normal. Calcitonin lowers calcium by inhibiting bone resorption but plays a much smaller role in adults, while thyroxine and insulin regulate metabolism and glucose rather than calcium balance.