FGF23 and chronic kidney disease: Despite high levels of FGF-23, only a subgroup of patients with ADPKD and CKD stages G1 and G2 develop kidney phosphate wasting, defined as a maximal tubular reabsorption ratio of phosphate to glomerular filtration ratio (TmP/GFR) of <0.8 mmol/L, with or without hypophosphatemia [4, 5].