Calcitriol or 1,25-dihydroxycholecalciferol (abbreviated 1,25-(OH)2-D3) is the active form of vitamin D found in the body (vitamin D3). Calcitriol is marketed under various trade names including Rocaltrol (Roche), Calcijex (Abbott) and Decostriol (Mibe, Jesalis). It is produced in the kidneys via 25-hydroxyvitamin D-1 α-hydroxylase by conversion from 25-hydroxycholecalciferol (calcidiol). This is stimulated by a decrease in serum calcium, phosphate (PO43−) and parathyroid hormone (PTH) levels. It regulates calcium levels by increasing the absorption of calcium and phosphate from the gastrointestinal tract, increasing calcium and phosphate reabsorption in the kidneys and inhibiting the release of PTH. Calcitriol is also commonly used as a medication in the treatment of hypocalcemia and osteoporosis.