The other endocrine glands are not directly controlled by the pituitary. The four parathyroid glands, located behind the thyroid, secrete a hormone that regulates calcium and phosphate metabolism. The endocrine portion of the pancreas, called the islets of Langerhans, secretes insulin, which regulates the level of sugar (glucose) in the blood and glucagon, which raises blood sugar level. The thymus, sometimes considered another endocrine gland, processes lymphocytes in newborn animals, seeding the lymph nodes and other lymph tissues; it is partly responsible for the development of the organism's immune system (see immunity). The kidney is sometimes considered an endocrine gland because it secretes the hormone renin which, with other substances, regulates blood pressure. The kidney produces a glycoprotein called erythropoietin, which stimulates red blood cell production. The pineal gland produces a substance called melatonin, which helps regulate the body's internal clock.