FGF23 and osteomalacia: X‐linked hypophosphatemia (XLH) is a rare, lifelong disease caused by loss of function mutations in the PHEX (phosphate‐regulating endopeptidase homolog, X‐linked) gene, resulting in increased circulating levels of FGF23 that lead to chronic hypophosphatemia and impaired production of 1,25‐dihydroxyvitamin D (1,25(OH)2D).1, 2 Low serum phosphorus levels result in rickets and osteomalacia, the hallmarks of XLH in children and adults, respectively.