KL and chronic kidney disease: Although FGF23 decreases the PTH level as a negative feedback mechanism under normal conditions [103,104], resistance to the action of FGF23 in both the parathyroid and kidney is found in advanced CKD due to downregulation of the FGF23 receptor complex, the klotho‒FGF receptor 1 (FGFR1) [105,106,107,108].