Hypophosphatemic rickets (previously called vitamin D-resistant rickets) is a disorder in which the bones become painfully soft and bend easily, due to low levels of phosphate in the blood. Symptoms usually begin in early childhood and can range in severity. Severe forms may cause bowing of the legs and other bone deformities; bone pain; joint pain; poor bone growth; and short stature. In some affected babies, the space between the skull bones closes too soon (craniosynostosis). This sometimes results in developmental abnormalities.

Hypophosphatemic rickets is almost always inherited and may be caused by changes (mutations) in any of several genes. There are several forms which are distinguished by their genetic cause and pattern of inheritance. Most commonly hypophosphatemic rickets is due to changes in the PHEX gene and inherited in an X-linked dominant manner (X-linked hypophosphatemia). Less commonly it is inherited in an X-linked recessive manner (often called Dent disease); autosomal dominant manner; or autosomal recessive manner. Treatment involves taking phosphate and calcitriol in order to raise phosphate levels in the blood and promote normal bone formation.

Most commonly, it is caused by a mutation in the PHEX gene. Other genes that can be responsible for the condition include the CLCN5, DMP1, ENPP1, FGF23, and SLC34A3 genes.

The genes associated with hereditary hypophosphatemic rickets are involved in keeping a proper balance of phosphate in the body. Many of these genes directly or indirectly regulate a protein that normally inhibits the kidneys' ability to reabsorb phosphate into the blood. Mutations affecting the function of these genes increase the production (or reduce the breakdown) of the protein, causing the protein to be overactive. The overactivity of the protein reduces phosphate reabsorption by the kidneys, leading to the features of the condition.

Rarer, sporadic, acquired cases are sometimes associated with benign (non-cancerous) mesenchymal tumors that decrease resorption of phosphate.

The symptoms of hypophosphatemic rickets usually begin in infancy or early childhood. Specific symptoms and severity can vary greatly among affected children. The condition can be so mild that there are no noticeable symptoms, or so severe that it causes bowing of the legs and other bone deformities; bone pain; joint pain; and short stature. Other symptoms may include premature closure of the skull bones in babies (craniosynostosis); limited joint movement; and dental abnormalities. If left untreated, symptoms worsen over time.

Because males have only one X chromosome (and one Y chromosome) and females have two X chromosomes, X-linked dominant conditions affect males and females differently. Both males and females can have an X-linked dominant condition. However, because males don't have a second, working copy of the gene (as females do), they usually have more severe disease than females.

If a father has the mutated X-linked gene:

• all of his daughters will inherit the mutated gene (they will all receive his X chromosome)
• none of his sons will inherit the mutated gene (they only inherit his Y chromosome)

If a mother has the mutated X-linked gene, each of her children (both male and female) has a 50% chance to inherit the mutated gene.

Less commonly, hypophosphatemic rickets is inherited in an X-linked recessive, autosomal dominant, or autosomal recessive manner.

The goal of treatment of adults with hypophosphatemic rickets is to manage bone pain and in some cases aid in the healing of bone fracture. Adults with hypoposphatemic rickets that are not experiencing symptoms may not benefit from therapy, but could experience therapy related complication (e.g., nephrocalcinosis and hyperparathyroidism). Treatment for symptomatic adults involves daily divided doses of Calcitriol and phosphate. Adults with hypophospatemic rickets undergoing treatment should carefully follow their prescribed dosing schedule. All adults with hypophosphatemic rickts should be regularly monitored for serum phosphorus, calcium, creatinine, and PTH.

If the condition is not treated (especially while children are growing), skeletal deformities may be permanent.