FGF23 and chronic kidney disease: Although FGF-23 measurements may provide a sensitive biomarker of abnormal renal phosphate handling in early CKD, it remains unknown how the osteocyte “senses” subtle abnormalities in renal tubular phosphate handling and whether a kidney-specific factor exists that triggers the release of FGF-23 from bone cells in response to transient elevations in serum phosphorus level or in response to unspecified disease processes in the kidney.