RTRs with vitamin D insufficiency , defined as circulating 25-hydroxyvitamin D levels of less than 30 ng/ml ( or 75 nmol/l ) , will be randomized between 12 and 48 months after transplantation to blinded groups to receive vitamin D3 ( cholecalciferol ) either at high or low dose ( respectively , 100,000 UI or 12,000 UI every 2 weeks for 2 months then monthly for 22 months ) with a follow-up of 2 years .