| Summary: | ABSTRACT: Objective: Fibroblast growth factor 23 (FGF23) dysregulation is implicated in the pathogenesis of hypophosphatemic disorders, including X-linked hypophosphatemic rickets, autosomal dominant hypophosphatemic rickets (ADHR), autosomal recessive hypophosphatemic rickets, and tumor-induced osteomalacia (TIO). Studies have suggested a role of iron deficiency in triggering FGF23 dysregulation in ADHR.Methods: We report a case of adult-onset FGF23-mediated hypophosphatemic osteomalacia with associated iron deficiency that had significant clinical improvement and reduction in FGF23 with iron replacement.Results: A 41-year-old female presented with progressively worsening muscle weakness and diffuse pain for 4 years, resulting in wheelchair dependence. She had hypophosphatemia, renal phosphate wasting, and an elevated FGF23. Extensive imaging for TIO was nonlocalizing. Family history was negative for bone disease. Despite phosphorus and calcitriol therapy, her phosphorus remained low and she had progressive weakness. She was noted to have iron deficiency, and initiation of iron replacement resulted in progressive clinical improvement, such that she was capable of ambulating for short distances unassisted after 6 months of iron replacement. Hypophosphatemia improved and FGF23 almost normalized after 50 weeks of iron therapy. Sequencing of the FGF23 gene was negative.Conclusion: Our case suggests that in addition to ADHR, iron deficiency may play a role in the pathophysiology of other FGF23-mediated disorders of renal phosphate wasting. Iron replacement may be a potential treatment option for such patients. Further studies will be needed to confirm this clinical observation.Abbreviations:ADHR = autosomal dominant hypophosphatemic ricketsFePi = fractional excretion of phosphorusFGF23= fibroblast growth factor 23
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