FGF23 and chronic kidney disease: Clinical studies in patients with CKD have shown that soluble klotho is lower than normal (519 ± 183 versus 845 ± 330 pg/mL, P < .0001) in renal patients, and soluble klotho is positively correlated with serum calcium and negatively correlated with serum phosphate, PTH, and FGF23, suggesting that soluble klotho might reflect the ensuing tubular resistance to FGF23, which could be an early marker of CKD-MBD [143, 144].