Which hormone helps in regulating the body's calcium levels?

Calcitriol is the active form of vitamin D and plays a crucial role in regulating the body's calcium and phosphorus levels. It is produced in the kidneys and acts to increase the absorption of calcium and phosphorus from the gastrointestinal tract, promote calcium reabsorption in the kidneys, and mobilize calcium from the bones when necessary. By enhancing the availability of calcium in the blood, calcitriol helps to ensure that various physiological processes that depend on calcium, such as muscle contraction and nerve transmission, function optimally.

Erythropoietin is primarily involved in stimulating red blood cell production rather than regulating calcium levels. Aldosterone is a hormone that helps regulate sodium and potassium levels and has an effect on blood pressure but does not play a direct role in calcium homeostasis. Insulin is mainly responsible for glucose metabolism and promoting the uptake of glucose by cells and does not have a significant role in calcium regulation.