FGF23 is secreted from bone tissue and binds to a complex of α-Klotho and FGFR1c, FGFR3c, or FGFR4 in kidney as a hormone to regulate systemic phosphate homeostasis and vitamin D metabolism.363 A secondary elevation of serum FGF23 levels is commonly detected in CKD patients that are partly due to decreased renal clearance.364 The increased FGF23 is beneficial for lowering serum phosphate level and reducing 1,25(OH)2D3, which further increases the PTH level. Here, KL is linked to chronic kidney disease.