What does serum fibroblast growth factor 23 do in hemodialysis patients?

Fibroblast growth factor 23 (FGF-23) is a phosphate-regulating substance largely produced by osteocytes. Its major action, in normal subjects, is the inhibition of Na-coupled reabsorption of inorganic phosphate in the renal proximal tubule. FGF-23 levels increase markedly in dialysis patients. Why do the FGF-23 levels increase in these patients, and do they have any physiologic or pathophysiologic effects?