FGF23 and stage 5 chronic kidney disease: To this end, we assessed the longitudinal association between plasma levels of 25(OH)D, 1,25(OH)2D, iPTH, and FGF-23 and cognitive function in 605 patients with advanced CKD not requiring dialysis (n = 247) and end-stage renal disease (ESRD; n = 358) who participated in the multi-center randomized controlled Department of Veterans Affairs Cooperative Study Program Homocysteine Study Cognitive Function Substudy (HOSTCOG).