What is FGF 23?
FGF 23 is classified as a phosphotonin—a hormone that increases urinary phosphorus. It is produced and released by osteocytes in response to increased serum phosphorus.
When FGF 23 binds its receptor and the receptor’s co-factor, Klotho, it decreases phosphorus reabsorption in the proximal tubule. It also decreases 1-alpha hydroxylase, lowering 1,25-OH D levels. FGF-23 also can directly suppress PTH. Increases in FGF-23 are found early in chronic kidney disease (CKD) and is responsible for the decreased 1,25-OH D levels found in CKD. It helps maintain normal phosphate levels despite compromised glomerular filtration rate (GFR).