The kidneys make two main hormones, vitamin D and erythropoietin.
Vitamin D is not a vitamin but a hormone essential for a number of different functions in the body. Most of the vitamin D that is in the blood is inactive and it is catalyzed by the kidney and other tissues to activate it. Active vitamin D helps in absorption of calcium from food, is important for the maintenance of healthy bones and also helps to regulate the response of the immune system to infection.
Erythropoietin is produced by the kidney and acts on bone marrow to stimulate the production of mature red blood cells.
The kidneys also produce prostaglandins, hormone-like substances, made from lipid (fat). The substances are one way in which the production of renin is stimulated. Renin is an enzyme, also produced by the kidneys, that plays an important role in the renin–angiotensin–aldosterone hormonal system, which helps to control blood pressure. In addition to making hormones, the kidneys also respond to a number of hormones including vitamin D, aldosterone, prostaglandins, cortisol, parathyroid hormone and calcitonin.