Which hormone is primarily responsible for regulating calcium levels in the blood?

The primary hormone responsible for regulating calcium levels in the blood is parathyroid hormone (PTH). PTH is produced by the parathyroid glands and plays a crucial role in maintaining calcium homeostasis. When blood calcium levels drop, PTH is secreted to elevate those levels by promoting the release of calcium from bones, increasing the absorption of calcium in the intestines, and enhancing renal tubular reabsorption of calcium, thus reducing its excretion in urine.

This regulation is vital because calcium is essential for various bodily functions, including nerve transmission, muscle contraction, and blood clotting. The other hormones listed, such as calcitonin, insulin, and glucagon, have different functions in the body. For example, calcitonin, which is produced by the thyroid gland, functions to lower blood calcium levels, opposing the action of PTH. Insulin and glucagon primarily regulate blood glucose levels rather than calcium. Therefore, parathyroid hormone is the key hormone involved in increasing and maintaining adequate calcium levels in the bloodstream.