FGF23 and chronic kidney disease: In patients with early stages of CKD, increased FGF-23 levels reduce 1,25(OH)2D3 formation and are likely to contribute prominently to maintaining serum phosphate levels within normal limits.(55,56) However, in these patients, FGF-23 also may have systemic “off-target” effects that contribute to cardiovascular disease, a leading cause of death in both adult and pediatric CKD patients.