Which of the following does not have an effect on plasma calcium levels?

• A. Parathyroid hormone
• B. Vitamin D
• C. Calcitonin
• D. Aldosterone

Answer: (D)

Calcium balance in the body is controlled mainly by three hormones that act on bone, kidney, and gut: parathyroid hormone, vitamin D, and calcitonin. Aldosterone does not have a direct role in regulating plasma calcium.

Parathyroid hormone responds to low calcium by increasing bone resorption, enhancing renal calcium reabsorption in the distal tubule, and stimulating the production of active vitamin D. Active vitamin D then increases intestinal calcium absorption, helping raise blood calcium levels.

Vitamin D in its active form directly boosts gut absorption of calcium (and phosphate), supporting adequate calcium availability, especially when dietary intake is low. It works alongside PTH to maintain calcium homeostasis.

Calcitonin helps keep calcium from getting too high by inhibiting osteoclast activity in bone, which reduces bone resorption and lowers plasma calcium.

Aldosterone mainly regulates sodium and potassium balance in the kidneys, with effects on water and electrolytes rather than a direct influence on calcium. Therefore it does not actively regulate plasma calcium levels.