Iron and fibroblast growth factor 23 in X-linked hypophosphatemia

•Intact FGF3 is more likely to be elevated than C-terminal FGF23 in XLH.•Despite hypophosphatemia, C-terminal FGF23 is inversely related to iron in XLH.•Intact FGF23 is not related to serum iron in XLH or controls.•The presented data suggest that FGF23 cleavage is a regulated process even in XLH.

BackgroundExcess fibroblast growth factor 23 (FGF23) causes hypophosphatemia in autosomal dominant hypophosphatemic rickets (ADHR) and X-linked hypophosphatemia (XLH). Iron status influences C-terminal FGF23 (incorporating fragments plus intact FGF23) in ADHR and healthy subjects, and intact FGF23 in ADHR. We hypothesized that in XLH serum iron would inversely correlate to C-terminal FGF23, but not to intact FGF23, mirroring the relationships in normal controls.MethodsSubjects included 25 untreated outpatients with XLH at a tertiary medical center and 158 healthy adult controls. Serum iron and plasma intact FGF23 and C-terminal FGF23 were measured in stored samples.ResultsIntact FGF23 was greater than the control mean in 100% of XLH patients, and > 2SD above the control mean in 88%, compared to 71% and 21% respectively for C-terminal FGF23. In XLH, iron correlated negatively to log-C-terminal FGF23 (r = − 0.523, p < 0.01), with a steeper slope than in controls (p < 0.001). Iron was not related to log-intact FGF23 in either group. The log-ratio of intact FGF23 to C-terminal FGF23 was higher in XLH (0.00 ± 0.44) than controls (− 0.28 ± 0.21, p < 0.01), and correlated positively to serum iron (controls r = 0.276, p < 0.001; XLH r = 0.428, p < 0.05), with a steeper slope in XLH (p < 0.01).ConclusionLike controls, serum iron in XLH is inversely related to C-terminal FGF23 but not intact FGF23. XLH patients are more likely to have elevated intact FGF23 than C-terminal FGF23. The relationships of iron to FGF23 in XLH suggest that altered regulation of FGF23 cleaving may contribute to maintaining hypophosphatemia around an abnormal set-point.