Description derivative of 7-dehydroxycholesterol formed by ultraviolet rays breaking of the c9-c10 bond. it differs from ergocalciferol in having a single bond between c22 and c23 and lacking a methyl group at c24indication for the treatment of vitamin d deficiency or insufficiency, refractory rickets (vitamin d resistant rickets), familial hypophosphatemia and hypoparathyroidism, and in the management of hypocalcemia and renal osteodystrophy in patients with chronic renal failure undergoing dialysis. also used in conjunction with calcium in the management and prevention of primary or corticosteroid-induced osteoporosis.