Effect of Calcitriol on FGF23 Level in Healthy Adults and its Dependence on Phosphate Level.

Aim: To evaluate the short-term effects of calcitriol and sevelamer hydrochloride on fibroblast growth factor-23 (FGF23) in humans and to determine whether the effect is direct or indirect through calcitriol-induced increased absorption of phosphorus from the intestine. Patients and Methods: A total of 15 healthy individuals were tested at three time points and stages, for 24 h and at 1-week intervals. During each stage, blood samples were taken at three time points (0, 8 and 24 h); baseline stage: under no intervention; second stage, while receiving 0.5 mu g calcitriol orally twice daily; and at the third stage, while receiving 0.5 mu g calcitriol orally twice daily and sevelamer hydrochloride during meals. The changes in FGF23, parathyroid hormone, calcitriol, Ca, and phosphorus were determined. Results: During calcitriol administration, the FGF23 level changed significantly (p=0.008), with the level at 24 h levels being significantly higher than at 8 h (8.8 pg/ml vs. 13.0 pg/ml, p=0.036). There was a statistically significant difference in the percentage change, among the three stages, at time 8 to 24 h and 0 to 24 h for FGF23 (p=0.014 and p=0.015, respectively), with significant differences between baseline vs. calcitriol for 8 to 24 h FGF23 change (-9.23% vs. 26.98%, p=0.003) and a trend between baseline vs. calcitriol (p=0.061) and calcitriol plus sevelamer (p=0.069) for 0 to 24 h FGF23 change. Conclusion: Administration of calcitriol to healthy individuals increases the circulating level of FGF23 within 24 h. Combined calcitriol and sevelamer administration restrains the increase of FGF23, suggesting that calcitriol-induced increased absorption of phosphate from the intestine might also be involved in the increase of FGF23.