Growth hormone is an off-label, yet theoretically attractive adjunctive therapy for XLH because it (transiently) increases phosphatemia and 1,25(OH)2D, lowers PTH and increases TmP/GFR, may improve height Z-scores without influencing body disproportion (158, 159) and might improve muscle strength. The gene discussed is GH1; the disease is X-linked hypophosphatemia.