FAQs About XLH

FAQs About XLH

Being diagnosed with rare genetic diseases, such as X-linked hypophosphatemia (XLH), can often leave patients and their caregivers worried and overwhelmed. To help alleviate some confusion, we present answers to some of the most frequently asked questions about XLH.

What is XLH?

XLH is a genetic disorder that occurs in childhood. It is characterized by lower-than-normal phosphate levels in the blood due to abnormal processing by the kidneys, which leads to phosphate wasting (loss of phosphate in the urine). This causes symptoms such as a form of rickets that’s characterized by slow growth, bent legs, dental problems, and muscle pain — symptoms that do not improve with vitamin D supplementation.

Why is phosphate important?

Phosphate contains phosphorus, the mineral required to build and repair bones and teeth, and for proper muscle contraction. Most of the body’s phosphorus is found in the bones while kidneys regulate the amount of phosphorus in the blood. An imbalance in phosphorus levels can result in improper bone mineralization and also affect other body functions.

How prevalent is XLH?

XLH is found in about 1 in 20,000 births. An estimated 3,000 children and 9,000 adults are thought to be affected by XLH in the U.S.

What causes XLH?

XLH is caused by mutations in the PHEX gene situated on the X-chromosome. This gene encodes for an enzyme that regulates another protein called fibroblast growth factor 23 (FGF23). FGF23 inhibits the kidneys from reabsorbing phosphate into the blood.

Excess FGF23 activity due to PHEX mutations causes the kidneys to excrete large amounts of phosphate in the urine, resulting in the symptoms of XLH.

How is XLH inherited?

XLH is inherited in an X-linked dominant manner so both males and females can be affected by the disease. A man affected by XLH will pass on the disease to all his daughters but not to his sons. A woman affected by the disease has a 50% chance of passing it on to any offspring.

Can XLH appear without a known family history?

Yes. About 20–30% of XLH cases are known to be caused by spontaneous mutations, and not by inheriting the disease from parents.

How do I know if my child has XLH?

The first signs of XLH appear in early childhood with symptoms such as slow growth, outward curvature of the legs when standing, knees touching each other at an angle (knock-knees), bone and joint pain, a waddling gait while walking, and pus in the teeth (dental abscesses).

Adult symptoms include bone, muscle, and joint pain, frequent fractures, early osteoarthritis, dental abscesses, and fatigue.

Blood and urine tests can be used to diagnose XLH. A genetic test can reveal known mutations in the PHEX gene and confirm the diagnosis.

Is XLH contagious?

XLH is a genetic disease and is not contagious.

What are the treatment options?

Treatment for XLH targets the symptoms and is aimed at relieving pain and correcting bone defects via surgery. A synthetic form of vitamin D called calcitriol is usually given to children along with phosphate supplements until their bone growth is completed.

Crysvita (burosumab) by Ultragenyx Pharmaceutical is a new treatment that was recently approved by the U.S. Food and Drug Administration for the treatment of XLH.

For more information, check out our page about XLH treatments.

Where can I find more information?

Our website provides information about XLH news, therapy, research, and clinical trials. Other sites, such as the XLH Network and XLH Link, offer networking opportunities, doctor information, and a variety of other resources for patients.


Last updated: Dec. 9, 2019


XLH News Today is strictly a news and information website about the disease. It does not provide medical advice, diagnosis, or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. 

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Özge has a MSc. in Molecular Genetics from the University of Leicester and a PhD in Developmental Biology from Queen Mary University of London. She worked as a Post-doctoral Research Associate at the University of Leicester for six years in the field of Behavioural Neurology before moving into science communication. She worked as the Research Communication Officer at a London based charity for almost two years.

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