FGF23 and chronic kidney disease: Although the heart may be an important target tissue at supraphysiological FGF23 concentrations, promoting cardiomyocyte hypertrophy in CKD patients by a Klotho-independent signaling pathway (67, 68), FGF23 is not expressed in the normal heart, and heart function is normal in Fgf23/VDR mutant mice (69).