What is XLH?

XLH – X-Linked Hypophosphatemia - is a genetic disorder that affects about one in 20,000 people. Typically, it is passed from one generation to the next, but sometimes appears in an individual with no family history of XLH.

XLH is carried on the X chromosome; hence the “X-Linked” in the name. Hypophosphatemia means “low level of phosphorus in the blood.”  Almost all X-linked disorders are recessive, but XLH is one of the few that is in fact dominantly inherited.  XLH is also the most common of the hypophosphatemic rickets disorders.

An XLH person’s kidneys do not properly handle vitamin D and phosphorus, but this is not caused by a kidney problem. Instead, something circulating in the bloodstream causes the kidneys to treat phosphorus as a waste product and not return enough of it to the circulation for use by bones and teeth.  Thus, a kidney transplant would not solve the problem. Likewise, if an XLH person’s kidney is transplanted into another person, that person will not develop XLH.

Knock-knees or bowing of the legs are the most noticeable symptoms of the rickets disorder that can occur as a result of XLH.  In adults, the bone disease is called osteomalacia, that is, soft bones.

Vitamin D-Resistant Rickets was the term first used for this disorder because XLH was initially recognized by the ineffectiveness of normal diets to prevent rickets. In the average person, normal diets and sunlight exposure contain enough Vitamin D to avoid simple rickets, but that doesn’t work for people with XLH. Their condition resists treatment with most forms of Vitamin D, although a more active form of this vitamin hormone is an important part of their treatment.

Today the syndrome is defined more by the root cause of the disorder, based on phosphorus metabolism, rather than on the role played by Vitamin D.

Other Names for XLH

• X-Linked Hypophosphatemia (XLH)
• X-Linked Hypophosphatemic Rickets
• Familial Hypophosphatemia or Familial Hypophosphatemic Rickets
• Vitamin D-Resistant Rickets (VDRR)
• Genetic Rickets

Source: The XLH Network Board of Directors

© 2011, The XLH Network Inc.
The authors of this web site are not medical professionals, and this information does not substitute for medical care. Information on these pages is based on biomedical research, published in peer-reviewed journals, and international research conferences. Additionally, in some cases anecdotal information is provided by subscribers of the F-HYPDRR group, a mailing list for The XLH Network Inc. A listing of XLH research is available. Please read our full disclaimer.  

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Last modified Apr 20, 2012