VDR and vitamin D deficiency: PTH synthesis and parathyroid gland proliferation are caused by a variety of factors in CKD patients, including hypocalcaemia, hyperphosphatemia, vitamin D deficiency, reduced density of vitamin D receptor (VDR) and calcium-sensing receptor (CaSR) and disturbance of fibroblast growth factor 23 (FGF23)-Klotho axis (Okazaki et al., 1988; Brown et al., 1993; Urakawa et al., 2006).