FGF23 and X-linked hypophosphatemia: However, the tubular maximum reabsorption of phosphate (TmP) per unit GFR (TmP/GFR) did not decrease despite excessive FGF23 secretion in the majority of ADPKD patients, whereas TmP/GFR decreased in X-linked hypophosphatemia patients whose serum FGF23 levels were lower than those in ADPKD patients [21].