FGF23 and chronic kidney disease: Oral administration delivers hormone directly to vitamin D receptors (VDR) localized in the upper small intestine, increasing active absorption of dietary calcium.3 Hormone therapy also increases serum phosphorus and fibroblast growth factor 23 (FGF23), which have been linked to faster CKD progression and induction of CYP24A1 expression in target tissues, decreasing therapeutic responsiveness.3