Which hormone, together with vitamin D, plays a dominant role in calcium regulation?

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

Answer: (B)

Calcium balance is controlled mainly by parathyroid hormone working in concert with vitamin D. When calcium levels drop, parathyroid hormone is released and it raises serum calcium through several coordinated actions: it stimulates bone resorption to release calcium into the blood, it increases calcium reabsorption in the kidneys, and it enhances the production of active vitamin D in the kidneys. The active vitamin D (calcitriol) then boosts intestinal absorption of calcium (and phosphate). This combination—PTH driving resorption and renal reabsorption plus stimulating calcitriol production, which increases gut absorption—produces a rapid and effective rise in calcium. Calcitonin can lower calcium, but it plays only a minor role in humans and is not the primary regulator. Insulin and glucagon regulate metabolism of glucose, not calcium homeostasis.