FGF23 and chronic kidney disease: However, with a reduction in tubular mass, both 1-α-hydroxylase and erythropoietin secretion should be decreased, and evidence shows that erythropoietin secretion remains unaffected in earlier stages of CKD when decreased calcitriol is present.105 Through the actions of FGF-23, serum phosphorus concentration is maintained at the expense of calcitriol, and the decrease in calcitriol is greater than would be predicted from the numbers of nephrons lost.