FGF23 and osteomalacia: In adults, the most frequently encountered causes of acquired phosphopenic osteomalacia are oncogenic osteomalacia (caused by ectopic FGF23 production), repeated i.v. iron administration,(8) and Fanconi syndrome—a generalized proximal renal tubular dysfunction characterized by urinary loss of glucose, phosphate, amino acids, and β2‐microglobulin, as well as a reduction in acid clearance.