Calcitriol is a form of Vitamin D3 with three alcohol groups. It increases gastrointestinal calcium absorption, stimulates osteoclastic calcium resorption from bone, facilitates the effect parathyroid hormone (PTH) has on bone resorption, and increases renal tubular absorption of calcium. Production of calcitriol by the cells of the proximal tubule of the nephron in the kidney is stimulated by hypocalcemia and hyperphosphatemia as well as parathyroid hormone. Types include: 1,25-dihydroxycholec… (More on Calcitriol)