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# FGF23-blocking antibodies

Antibodies that neutralise FGF23, the hormone that makes the kidney waste phosphate. Phosphate rises and rickets heals.

Updated Nov 1, 2026

## What they are

Monoclonal antibodies against fibroblast growth factor 23 (FGF23). Too much FGF23 causes X-linked hypophosphataemia (XLH) and tumour-induced osteomalacia: phosphate is lost in the urine, active vitamin D is low, and bones become soft (rickets in children, osteomalacia in adults). Burosumab is the member in use.

## Mechanism of action

FGF23, made by bone cells, acts on the kidney to reduce phosphate reabsorption in the proximal tubule and to suppress the enzyme that makes active vitamin D (calcitriol). Neutralising FGF23 restores tubular phosphate reabsorption and calcitriol production, so serum phosphate rises and bone mineralisation improves.

Target: Fibroblast growth factor 23 (FGF23), a phosphate-regulating hormone from bone

## Key points

- XLH is caused by excess FGF23 (PHEX gene); the antibody treats the cause instead of replacing phosphate several times a day.
- Oral phosphate and active vitamin D (calcitriol, paricalcitol) are stopped before starting.
- Not used when serum phosphate is already normal or high, or in severe kidney disease.
- Serum phosphate is checked to avoid overshoot; nephrocalcinosis has been reported.

## Drugs in this class

- [**Burosumab** An antibody injection against FGF23. It stops phosphate loss in the urine and heals rickets in X-linked… Fibroblast growth factor 23 (FGF23)](https://durgical.com/en/pharmacology/burosumab/)

## Test yourself

Think of the answer, then tap the question to see it.

## What are the two kidney actions of FGF23 that an anti-FGF23 antibody reverses?

Reduced proximal tubular phosphate reabsorption and suppressed calcitriol synthesis.

## Why are oral phosphate and calcitriol stopped before burosumab?

Combined with the antibody they could raise phosphate too much (hyperphosphataemia and mineral deposits).

## Which inherited disease is the main indication for FGF23 antibodies?

X-linked hypophosphataemia (XLH).

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Burosumab-twza Injection: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a618034.html) (MedlinePlus (U.S. National Library of Medicine), 2026).

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