FGF-23 (fibroblast growth factor 23)
A hormone released by bone that controls phosphate. It tells the kidneys to get rid of phosphate and lowers active vitamin D. It is measured mainly to work out rare phosphate-wasting disorders that cause soft bones, and it rises early in kidney disease.
What it measures
FGF-23 is a hormone made by bone cells (osteocytes) whose job is to keep blood phosphate in check. When phosphate is high, FGF-23 rises and acts on the kidneys to excrete more phosphate in the urine, and it lowers the active form of vitamin D — together bringing phosphate down. It works in a system alongside parathyroid hormone (PTH) and vitamin D to regulate phosphate, calcium and bone mineralisation. Measuring FGF-23 has two main uses. First, in rare disorders where too much FGF-23 causes the kidneys to waste phosphate, blood phosphate falls and bones fail to mineralise properly — causing rickets in children or osteomalacia (soft, aching bones) in adults; these include inherited conditions (like X-linked hypophosphataemic rickets) and tumour-induced osteomalacia, and FGF-23 is key to diagnosing them. Second, FGF-23 is one of the earliest markers to rise in chronic kidney disease, climbing before phosphate itself becomes abnormal as the body tries to keep phosphate normal, and high levels are linked to worse cardiovascular and kidney outcomes. It is a specialist test, interpreted alongside phosphate, calcium, PTH, vitamin D and kidney function.
What a high value can mean
- Phosphate-wasting disorders — inherited hypophosphataemic rickets, or tumour-induced osteomalacia; cause low phosphate and soft bones.
- Chronic kidney disease — rises early, before phosphate becomes abnormal; linked to worse outcomes.
- High dietary or supplemental phosphate load.
- Certain iron formulations — some intravenous irons can raise FGF-23.
What a low value can mean
- Normal or low — the expected result outside these specific disorders.
- Rare disorders of too little FGF-23 activity — cause the opposite, high phosphate with calcification (tumoral calcinosis).
When to discuss with a doctor
FGF-23 is a specialist test rather than a routine one. It is most useful when a low blood phosphate with soft, aching bones suggests a phosphate-wasting disorder: a high FGF-23 points to an FGF-23-driven cause (inherited or, importantly, a phosphate-wasting tumour), which changes treatment. In chronic kidney disease it is used mainly in research and specialist assessment as an early marker of disturbed mineral metabolism. It is always interpreted alongside phosphate, calcium, PTH, vitamin D and kidney function, since these hormones work as a system. Mediora.AI shows FGF-23 in the context of phosphate and the other mineral markers so the pattern is visible; the interpretation belongs with your specialist team.