What is the primary hormone that regulates calcium levels in the blood?

The primary hormone that regulates calcium levels in the blood is parathyroid hormone (PTH). PTH is secreted by the parathyroid glands and plays a crucial role in maintaining calcium homeostasis. When calcium levels in the blood are low, PTH is released, which stimulates several processes to increase calcium concentration. These include promoting the release of calcium from bones into the bloodstream, enhancing the absorption of calcium from the intestines (with the help of vitamin D), and reducing the loss of calcium in urine by acting on the kidneys. This hormone ensures that calcium levels remain within a narrow range, which is essential for various physiological processes, including bone health, muscle function, and nerve transmission.

Other hormones like cortisol, insulin, and aldosterone do not primarily regulate calcium levels. Cortisol is involved in the stress response and metabolism; insulin regulates blood glucose levels; and aldosterone is crucial for sodium and potassium balance. Therefore, their roles do not directly affect calcium homeostasis in the same way that PTH does.