In primary hyperparathyroidism (PHPT), an enlargement of one or more of the parathyroid glands causes overproduction of the parathyroid hormone. This hormone regulates the level of calcium in the body, including the release of calcium from bones and the excretion of calcium in the urine. It also increases calcium absorption in the duodenum by changing 25(OH)D to the active form of vitamin D: 1,25(OH)2D. Abnormally high parathyroid hormone levels typically result in hypercalcemia.