X-linked hypophosphatemia (XLH) is a genetic disorder that causes low levels of phosphorus in the blood and affects bones, muscles, and teeth. XLH causes an excess amount of phosphate to be eliminated by the kidneys in the urine (otherwise known as phosphate wasting or hypophosphatemia). Phosphate is a mineral that is important for the normal formation of bones and teeth. In patients with XLH, fibroblast growth factor 23 (FGF23) levels are increased.
FGF23 functions to regulate the amount of phosphorus in the body. Increased levels of FGF23 prevent the body from retaining enough phosphate and result in low phosphorus levels. Over time, low phosphorus levels lead to soft, weak bones that are more likely to fracture. Bones in the hips, legs, and feet are more prone to fracture which can result in pain and difficulty moving.
Typically the disease is inherited. “X”-linked means that the disease is due to a defect in the PHEX gene on the X chromosome and the condition can be passed to offspring. In some cases, the disease is not inherited from a parent and is due to a new defect in the gene.
XLH can also be referred to as other names including hereditary hypophosphatemic rickets, vitamin-D resistant rickets, or genetic rickets.