Sample Types: serum, plasma. Range: 0.2 - 20 pmol/L. Sensitivity: 0.1 pmol/L. Time: Overnight ? FGF23 (fibroblast growth factor 23) is a 32 kDa protein with 251 amino acids that is proteolytically processed between arginine179 and serine180 to generate N-terminal and C-terminal fragments. FGF23 is mainly secreted by osteocytes and controls phosphate and 1,25(OH)2 vitamin D homeostasis.