PTH and chronic kidney disease: Briefly, circulating α-klotho levels decrease first, then serum fibroblast growth factor 23 (FGF23) levels increase, and serum calcitriol levels decrease in CKD stages 2 and 3, and serum parathyroid hormone (PTH) levels increase, followed by an increase in serum phosphate levels and a decrease in serum calcium levels [5].