However, raised FGF23 cannot definitively establish the diagnosis of TIO since there is a considerable overlap in FGF-23 levels between TIO and inherited conditions of hypophosphataemia [42], TIO-like syndromes [46], recent renal transplantation [47], and drug-related hypophosphatemia, mainly due to intravenous iron [48–50] Still, FGF23 levels are generally higher in TIO than in XLH and other causes of hypophosphatemia [42]. The gene discussed is FGF23; the disease is Down syndrome.