Elevated FGF23 in a patient with hypophosphatemic osteomalacia associated with neurofibromatosis type 1

Abstract Context The mechanism behind hypophosphatemia in the setting of neurofibromatosis type 1 (NF1) is not known. We describe a possible role of fibroblast growth factor 23 (FGF23) in the pathophysiology of hypophosphatemia in a patient with NF1. Case description A 34-year woman with NF1 presented with severe hypophosphatemia, osteomalacia, and elevated plasma FGF23. The patient had considerable improvement on replacement of oral phosphate. Two Ga68 DOTANOC PET-CT scans over a period of 2 years failed to detect any localized uptake. Immuno-staining for FGF23 was absent in the neural-derived tumour cells of the neurofibromas in the proband. Conclusion The patient with NF1 had elevated had elevated circulating FGF23. Tumour cells in the neurofibroma tissues did not stain for FGF23 on IHC. It is unlikely for neurofibromas to contribute to high circulating FGF23 levels in the proband.