FGF23 and hyperphosphatemia: Burosumab, a recombinant human IgG1 monoclonal antibody against FGF23, led to hyperphosphatemia after one dose of 60 mg requiring dose downtitration (Figure 3B), and TmP/GFR normalized after 3 doses (1.14 mmol/L [0.84-1.23 mmol/L]).