vitamin D

vitamin D A fat-soluble vitamin occurring in the form of two steroid derivatives: vitamin D₂ (ergocalciferol, or calciferol), found in yeast; and vitamin D₃ (cholecalciferol), which occurs in animals. Vitamin D₂ is formed from a steroid by the action of ultraviolet light and D₃ is produced by the action of sunlight on a cholesterol derivative in the skin. Fish-liver oils are the major dietary source. The active form of vitamin D is manufactured in response to the secretion of parathyroid hormone, which occurs when blood calcium levels are low. It causes increased uptake of calcium from the gut, which increases the supply of calcium for bone synthesis. Vitamin D deficiency causes rickets in growing animals and osteomalacia in mature animals. Both conditions are characterized by weak deformed bones.