FGF23 and hypophosphatemia: At the start of burosumab treatment, laboratory assessments showed hypophosphatemia (3.1 mg/dL, age specific reference range 3.7 – 5.6 mg/dL for children 4–11 years old) (23), increased FGF23 level (251 pg/mL), ALP (2118 IU/L) and BALP levels (942 μg/L), normal 25OHD (58 ng/mL) and PTH levels (38 pg/mL), and reduced TmPO4/GFR values (2.8 mg/dL).