Cross-sectional analysis of abnormalities of mineral homeostasis, vitamin D and parathyroid hormone in a cohort of pre-dialysis patients. The chronic renal impairment in Birmingham (CRIB) study.

Background: Disturbances in mineral and vitamin D metabolism, which affect parathyroid hormone (PTH) synthesis, are well recognized in patients receiving dialysis. However, it is unclear at what stage of chronic kidney disease (CKD) these abnormalities develop.

Methods: The associations between CKD stages 3 and 5, and alterations of calcium, phosphate, vitamin D and PTH concentrations were assessed in 249 patients (mean age 61 years, 66% male) and 79 age- and sex-matched healthy controls.

Results: As compared to controls, serum phosphate concentrations were elevated among CKD patients (1.40 vs. 1.11 mmol/l; p < 0.0001). And levels of both 25-hydroxyvitamin D (42.1 vs. 60.4 nmol/l; p < 0.0001) and 1,25-dihydroxyvitamin D (58.2 vs. 119.5 pmol/l; p < 0.0001) were lower among patients with CKD, even among those with only stage 3 CKD and despite 73% of patients receiving vitamin D supplements. The ratio of 1,25-dihydroxy- to 25-hydroxyvitamin D was lower than controls, even among patients with stage 3 CKD (p = 0.0001), and this ratio diminished with advancing renal impairment. Concomitant elevations were observed in intact PTH (13.8 vs. 4.2 pmol/l; p < 0.0001) and whole PTH (7.9 vs. 2.7 pmol/l; p < 0.0001).

Conclusion: Impaired conversion of 25-hydroxy- to 1,25-dihydroxyvitamin D is an early feature of renal disease, and progresses as renal function deteriorates.