FGF23 and chronic kidney disease: However, in advanced CKD (stages 4–5) the serum P concentration may be increased [2] because the phosphaturic effects of FGF23 and PTH are not sufficient to overcome the marked reduction in the glomerular filtration of P. Consequently, at some stage of CKD, a reasonable strategy would be to reduce the intestinal absorption of P in an attempt to prevent the development of hyperphosphatemia.