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Synonym(s) PAH deficiency
Phenylalanine hydroxylase deficiency
Prevalence 1-5 / 10 000
Inheritance Autosomal recessive
Age of onset Infancy
  • E70.0
  • E70.1
  • C0031485
  • D010661
  • 10034872


Disease definition

Phenylketonuria (PKU) is the most common inborn error of amino acid metabolism and is characterized by mild to severe mental disability in untreated patients.


The prevalence of PKU shows considerable geographic variation. It is estimated to be 1/10,000 live births in Europe with a higher rate in some countries (Ireland, Italy). Prevalence is particularly high in Turkey: 1/4,000 live births. PKU is far rarer in the Finnish, African and Japanese populations.

Clinical description

In the absence of neonatal diagnosis, symptoms develop within a few months of birth, may be very mild to severe and include gradual developmental delay, stunted growth, microcephaly, seizures, tremors, eczema, vomiting, and musty odor. Untreated patients subsequently develop intellectual disability, behavioral disorders (hyperactivity) and motor disorders. Patients often have fair coloring as a result of tyrosine deficiency. The most common form of the condition is known as classical phenylketonuria (see this term) and is characterized by severe symptoms. A mild form has also been described (mild PKU; see this term), and an even milder form known as mild hyperphenylalaninemia (mild HPA or non-PKU HPA; see this term). A subset of patients with milder phenotypes has been found to be responsive to tetrahydrobiopterin (BH4), the cofactor of phenylalanine hydroxylation (BH4-responsive HPA, see this term).


PKU is caused by a wide range of mutations in the PAH gene (12q22-q24.2) coding for phenylalanine hydroxylase. Non-PAH mutations have been reported to cause a disorder known as hyperphenylalaninemia due to BH4 deficiency (see this term). Mutation frequency varies among different ethnic groups. Lower levels or absence of the phenylalanine hydroxylase enzyme underlie the clinical manifestations, as a result of toxic accumulation of phenylalanine in the blood and brain.

Diagnostic methods

The disorder is usually diagnosed through neonatal screening programs.

Differential diagnosis

PKU should be distinguished from BH4 deficiency.

Genetic counseling

Transmission is autosomal recessive. Genetic counseling should be provided to affected families.

Management and treatment

The mainstay of treatment is a low-phenylalanine diet and amino acids mixture for the forms that require treatment. The recommended maintenance level is usually between 120 and 360 micromol/L in newborns, with treatment considered essential in older patients with levels above 600 micromol/L. There is, however, no consensus concerning the level of phenylalanine above which treatment should be initiated and recommendations vary from country to country.


Prognosis is variable.

Expert reviewer(s)

  • Pr Nenad BLAU

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Detailed information

Summary information
Emergency guidelines
Clinical practice guidelines
Guidance for genetic testing
Article for general public
Clinical genetics review
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