It is not unusual for people with chronic kidney failure to have low levels of Vitamin D. Your doctor may check your Vitamin D level and if it is low, order a supplement. The amount prescribed to rebuild Vitamin D levels may sound like an enormous amount — a typical dose is 50,000 IU ergocalciferol (Vitamin D2) once a week for 6 to 12 weeks. Very large doses such as this are needed to rebuild stores of Vitamin D when there is a deficiency. Healthy kidneys are rich with Vitamin D receptors and play a major role in turning Vitamin D into its active form, 1,25-dihydroxyVitamin D or calciferol. When Vitamin D is in its active form, it helps balance calcium and phosphorus in your body by controlling absorption of calcium and phosphorus from the food you eat and regulates parathyroid hormone (PTH). When kidneys fail, their ability to activate Vitamin D is lost. Without the activated Vitamin D to control calcium and phosphorus levels in the blood, parathyroid hormone (PTH) will try to over compensate and go out of range.