PTH and chronic kidney disease: In this context, FGF-23 reduces phosphate reabsorption in the proximal tubules and suppresses the production of 1,25-dihydroxyvitamin D. However, in chronic kidney disease (CKD), FGF-23 levels rise early, often before overt changes in phosphate or parathyroid hormone (PTH) levels, representing one of the first signs of disrupted mineral metabolism and bone mineral disease [124].