THE ACTIVE forms of vitamin D—calcitriol, paricalcitol, and doxercalciferol—which also are known as vitamin D receptor activators (VDRAs), have been largely used in nephrology only as a treatment for secondary hyperparathyroidism, an elevation in parathyroid hormone (PTH) due to renal failure. Serum levels of calcitriol, the native form of active vitamin D, decline as renal function worsens, and calcitriol deficiency is almost universal among untreated patients on dialysis.