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

The hormone primarily responsible for regulating calcium levels in the body is parathyroid hormone. This hormone is produced by the parathyroid glands and serves a crucial role in maintaining calcium homeostasis. When blood calcium levels drop, parathyroid hormone is released into the bloodstream. It acts on various target organs, including the bones, kidneys, and intestines, to increase calcium levels in the blood.

In the bones, parathyroid hormone stimulates osteoclast activity, which leads to the release of calcium stored in bone tissue into the bloodstream. In the kidneys, it enhances the reabsorption of calcium, preventing its loss in urine, and also promotes the conversion of vitamin D to its active form, which increases calcium absorption in the intestines.

Together, these actions effectively raise blood calcium levels, thereby ensuring that the body maintains sufficient calcium for critical physiological functions such as nerve conduction, muscle contraction, and blood clotting. This intricate regulation underlines parathyroid hormone's pivotal role in calcium homeostasis.