Which hormone helps to maintain calcium levels in the blood?

Calcitriol, which is the active form of vitamin D, plays a crucial role in maintaining calcium levels in the blood. It helps increase the intestinal absorption of calcium from the diet and also promotes the reabsorption of calcium in the kidneys, minimizing its loss in urine. Additionally, calcitriol supports the release of calcium from bones into the bloodstream, thereby helping to raise blood calcium levels when they are low.

The importance of calcitriol is underscored by its interactions with other hormones that regulate calcium homeostasis, such as parathyroid hormone (PTH). When blood calcium levels drop, PTH is secreted, which in turn stimulates the production of calcitriol in the kidneys, enhancing its effects and ensuring that calcium levels are adjusted appropriately.

Other hormones listed, such as EPO (erythropoietin), primarily regulate red blood cell production; renin is involved in blood pressure regulation and fluid balance; and T3 (triiodothyronine) focuses on metabolic rate and energy regulation, none of which have a direct role in calcium metabolism compared to calcitriol.