PTH and osteoporosis: In the kidney, PTH enhances the reabsorption of calcium and promotes the expression of 25-hydroxyvitamin D-1α-hydroxylase, which increases the production of calcitriol.115 Calcitriol is well known for elevating serum calcium levels by enhancing calcium absorption in the intestine, release from the bone, and reabsorption in the kidney.114 Owing to the pivotal roles of PTH, osteoporosis frequently occurs in patients with primary hyperparathyroidism.