ESPE Abstracts

Background: Fibroblast growth factor 23 (FGF23) decreases renal phosphate reabsorption and serum 1,25-dihydroxyvitamin D [1,25(OH)2D] levels. X-linked hypophosphatemic rickets (XLH) is caused by mutations in the PHEX gene and accompanied by decreased serum inorganic phosphate (IP) and elevated serum FGF23 levels. Patients with XLH are generally treated with oral active vitamin D and phosphate, but some previous reports indicated that serum FGF23 levels increased with this treatment. However, biochemical parameters associated with serum FGF23 levels during the treatment in XLH patients remain unclear.

Objective and hypotheses: We analyzed biochemical parameters associated with serum intact FGF23 levels in treated XLH patients to obtain better outcomes.

Method: Sixteen patients (male 12, female 4) with XLH and normal kidney function were included. The mean age at the first visit was 8 months, and the mean observation period was 5.6 years. We used all the data measured or obtained during the observation period as follows. We examined the association of serum intact FGF23 levels with age, doses of active vitamin D (alphacalcidol) and phosphate, serum calcium (Ca), IP, alkaline phosphatase (ALP), intact parathyroid hormone (PTH), 1,25(OH)2D, and creatinine (Cr) levels in the XLH patients before and during the treatment using linear mixed-effects models in SPSS ver.23 software as statistic analysis.

Results: Serum FGF23 levels in the XLH patients were positively associated with serum Ca (P=0.000), IP (P=0.000), 1,25(OH)2D (P=0.001) and Cr (P=0.001) and negatively with age (P=0.000) and ALP (P=0.007).

Conclusion: These results indicate that Ca might induce FGF23 production as well as IP and 1,25(OH)2D. We might be careful not to overcorrect serum Ca, IP and 1,25(OH)2D levels during the treatment of XLH patients because increased FGF23 could exacerbate bone phenotypes and biochemical parameters. Considering the association of intact FGF23 with age and serum Cr levels in XLH patients, age and serum Cr levels could be involved in serum FGF23 levels, although their mechanisms are unclear.