Introduction

X-linked hypophosphatemia (XLH) is a rare, lifelong disorder caused by loss-of-function variants in the phosphate-regulating endopeptidase, X-linked (PHEX) gene, which results in excessive fibroblast growth factor 23 (FGF23) activity. Chronic hypophosphatemia in adults causes osteomalacia, leading to pseudofractures, osteoarthritis, enthesopathy, and severe musculoskeletal pain and stiffness [1]. Burosumab, a fully human IgG1 monoclonal antibody that targets FGF23, has significantly improved phosphate metabolism, fracture healing, and physical function in adults with XLH [2]. Long-term studies have shown these benefits persist [3]. In contrast, conventional therapy is associated with complications, such as nephrocalcinosis and secondary/tertiary hyperparathyroidism. A recent large cohort study in Asian patients found that hyperparathyroidism and renal dysfunction are significant comorbidities in those receiving conventional therapy [4]. However, treating XLH during pregnancy remains challenging. The most recent international clinical practice guidelines recommend conventional therapy during pregnancy, recognizing that data on burosumab safety in this context are insufficient [5, 6]. However, discontinuing burosumab may cause rapid recurrence of debilitating symptoms and compromise maternal well-being. Here, we present a case of a woman with XLH who received burosumab throughout pregnancy and discuss the clinical implications, particularly regarding the interpretation of markedly elevated FGF23 levels observed in the neonate.