FGF23 and mineral metabolism in the early post-renal transplantation period.

Background: The relationship between fibroblast growth factor 23 (FGF23) and vitamin D production and catabolism post-renal transplantation has not been characterized.

Methods: Circulating creatinine, calcium, phosphorus, albumin, parathyroid hormone, FGF23, and 1,25(OH)2 vitamin D (calcitriol) values were obtained pre-transplantation, daily post-operatively for 5 days, and at 6 months post-transplantation in 44 patients aged 16.4 ± 0.4 years undergoing renal transplantation at UCLA from 1 August 2005 through to 30 April 2007. 25(OH) Vitamin D and 24,25(OH)2 vitamin D concentrations were obtained at baseline and on post-operative days 5 and 180, and urinary concentrations of creatinine, phosphorus, and FGF23 were measured on post-operative days 1, 3, 5, and 180.

Results: Circulating phosphate concentrations declined more rapidly and the fractional excretion of phosphorus was higher in the first week post-transplantation in subjects with higher FGF23 values. Fractional excretion of FGF23 was low at all time-points. Circulating 1,25(OH)2 vitamin D levels rose more rapidly and were consistently higher in patients with lower FGF23 values; however, 25(OH) vitamin D and 24,25(OH)2 vitamin D values were unrelated to FGF23 concentrations.

Conclusions: Inhibition of renal 1α-hydroxylase, rather than stimulation of 24-hydroxylase, may primarily contribute to the relationship between FGF23 values and calcitriol. The rapid decline in FGF23 levels post-transplantation in our patient cohort was not mediated solely by the filtration of intact FGF23 by the new kidney.