FGF23 and chronic kidney disease: As CKD progresses, Klotho concentration continues to decline, causing FGF23 resistance and, therefore, leading to marked increases in serum FGF23 (CKD G2–G5) and PTH levels (CKD G3a–G5), as well as decreases in serum 25-hydroxyvitamin D (25(OH)D) levels (CKD G2–G5) [7–9].