What is XLH?

X-Linked Hypophosphatemia (XLH) is a rare genetic disorder that affects about one in 20,000 people. The key characteristic of XLH is a low level of phosphorus in the blood, called Hypophosphatemia. When a person is affected by XLH, their kidneys do not properly handle Vitamin D and phosphorus. This causes a variety of symptoms and disorders that usually affect their bones and teeth.

What causes XLH?

The key characteristic of XLH--low phosphorus in the blood—is the result of a mutation that inactivates one of the genes on the X Chromosome (hence the “X” in XLH). That means XLH is transferred genetically. However, XLH can appear in an individual with no known family history.

What are the symptoms of XLH?

Patients with XLH experience abnormal bone and tooth development. Many develop rickets, presenting with knock-knees or bowing of the legs. Other symptoms include:

  • Waddling gait
  • Short stature or declining growth rate
  • Spontaneous tooth abscesses
  • Bone pain, muscle pain and weakness

What do I need to know about XLH diagnosis?

Symptoms of XLH can be apparent before a child begins to walk. But as a rare genetic condition, XLH is often undiagnosed or misdiagnosed as simple Vitamin D deficiency. In the average person, normal diets and sunlight exposure successfully treat Vitamin D deficiency and conditions like rickets, but this doesn’t work for people with XLH.

Early diagnosis is extremely important for effective XLH treatment. The XLH Network is committed to helping medical professionals diagnose XLH, and to empowering XLH-affected individuals and their families to successfully advocate for their diagnosis.

Sample reference ranges for phosphorus by age

Source: PEDIATRIC REFERENCE RANGES, 3rd edition, by Steven J. Soldin, published by AACC Press, Washington, DC, 1999 pages 145-146

2.0-5d (<2.5 kg)504.6-8.01.50-2.60504.6-8.01.50-2.60
Specimen:1., 2., 3., Serum or Plasma
1. Soldin SJ, Hicks JM, Bailey J, et al. Pediatric reference ranges for phosphate on the Hitachi 747 analyzer. Clin. Chem. 1997; 43:S198. (Abstract)
2. Lockitch G, Halstead AC, Albersheim S, et al. Age and sex specific pediatric reference intervals for biochemistry analytes as measured with the Ektachem 700 analyzer. Clin. Chem 1988;34:1622-5.
3. Soldin SJ, Hunt C, Hicks JM. Pediatric reference ranges for Phosphorus on the Vitros 500 Analyzer. Clin Chem 1999;45:A22 (Abstract)
1. Hitachi 747 using ammonium molybdate method (Boehringer-Mannheim, Diagnostics, Indianapolis, IN).
2., 3. Ektachem 700 (2) and 500 (3) using ammonium molybdate method (Johnson & Johnson, Rochester, NY).
1., 3. Study used hospitalized patients and a computerized approach to removing outliers. Values are 2.5 - 97.5th percentiles.
2. Study used normal healthy children. Values are 2.5 - 97.5th percentiles.

(In this table, n is the number of subjects assessed in each age category.)

In comparing phosphorus levels to the charts below, it may be necessary to make a conversion. To convert mg/dL to/from mmol/L, SIGMA provides an inorganic phosphorus reference standard:
20mcg per ml equivalent to 645 nanomoles per ml
Since a deciLitre is 100 ml, this is the same as saying:
2000mcg/dL or 2mg/dL or 645 mcmoles/L or 0.645mmol/L
Therefore, 1mg/dL is the same as 0.322 mmol/L

These charts provide a reference tool for phosphorus levels by age as a helpful tool to successfully diagnose XLH.

Is there a cure for XLH? Is it treatable?

XLH is not curable, but it is treatable. XLH is treated with a combination of medication and surgical or orthopedic treatment, depending on the case. The network strongly suggests relying on specialists familiar with the condition and experienced in treating and managing it. Find a doctor near you.

To learn more about XLH, view our XLH fact sheet, download our brochure, and visit our Resources page