MP359
FGF23 AND MARKERS OF MINERAL METABOLISM IN SUBJECTS WITH PRESERVED RENAL FUNCTION

Introduction and Aims: FGF23 is a bone-derived hormone that regulates phosphate homeostasis. FGF23 is elevated in CKD and independently associated with poor renal and cardiovascular outcomes and mortality. The study of FGF23 in subjects with normal renal function has received little attention thus far.

Methods: We examined in a large, population-based study with 1128 participants the associations of FGF23 with markers of mineral metabolism and renal function.

Results: Median eGFR of the cohort was 98.2 ml/min per 1.73m², median FGF23 was 78.5 RU/ml. FGF23 increased and 1,25-OH Vitamin D₃ decreased significantly at an eGFR threshold of 90.2 and 88.9 ml/min per 1.73m², respectively. In contrast, we observed no threshold for PTH. PTH increased continuously with falling GFR. In multivariable analysis adjusting for sex, age, BMI and GFR, FGF23 was negatively associated with 1,25-OH Vitamin D₃, urinary absolute and fractional calcium excretion but not with serum calcium or PTH_.We observed a positive association of FGF23 with plasma phosphate, but no association with urinary absolute or fractional phosphate excretion and unexpectedly, a positive association with TmP/GFR.

Conclusions: In the absence of CKD, PTH increases earlier than FGF23 when GFR declines. The rise of FGF23 occurs at a higher GFR threshold than previously reported and is closely associated with a decrease of 1,25-OH Vitamin D₃. Furthermore, our analysis suggests that the main demonstrable effect of FGF23 in the setting of preserved renal function is suppression of 1,25-OH Vitamin D₃ rather than stimulation of renal phosphate excretion.