Which hormone is a regulator of calcium in the body?

• A. PTH
• B. Aldosterone
• C. Testosterone
• D. Insulin

Answer: (A)

Calcium levels in the blood are controlled mainly by parathyroid hormone. When serum calcium drops, the parathyroid glands release PTH. This hormone raises calcium in three ways: it stimulates bone to release calcium into the bloodstream, increases calcium reabsorption in the kidneys so more is conserved rather than excreted, and promotes the formation of active vitamin D, which boosts calcium absorption from the gut. The combined effect is an increase in circulating calcium.

The other hormones listed do not regulate calcium in this direct, integrated way. Aldosterone mainly controls sodium and potassium balance and fluid volume, not calcium. Testosterone influences bone density more broadly rather than acute calcium regulation. Insulin primarily governs glucose metabolism, with limited direct role in maintaining serum calcium.