PTH and chronic kidney disease: Numerous observational studies have associated low vitamin D status with higher parathyroid hormone (PTH) concentrations [5–8], and observational data have reported an inverse relationship between vitamin D status and multiple adverse health-related outcomes, including decreased bone mineral density [9, 10], faster CKD progression [11, 12], acceleration of vascular calcifications [13], progression of allograft dysfunction [14, 15], and increased risk of cardiovascular morbidity and mortality [16–18].