FGF23 and chronic kidney disease: Large observational studies in patients with CKD or AF and in the general population have found an association between elevated FGF23 and increased risk of developing AF.89, 138 High FGF23 and low αKlotho levels are associated with periods of AF in patients with paroxysmal or persistent AF.91 Increased expression of FGF23, FGFR4 mRNA, and FGFR4 protein in the right atrial appendages of patients with AF has been reported and positively correlate with atrial collagen fraction.