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Classic phenylketonuria

(Classic • fen-l-keet-n-UR-ee-uh)

General Condition Information

Other Names

  • Phenylketonuria
  • Phenylketonuria (Classic)

Condition Type

Birth Prevalence

  • Hundreds of babies are born with this condition each year in the United States. 
  • Visit GeneReviews to learn more about how often this condition occurs.

Screening Finding

Increased phenylalanine +/- increased ratio of phenylalanine to tyrosine

What is classic phenylketonuria

Classic phenylketonuria (PKU) is an inherited (genetic) condition that prevents the body from processing proteins correctly. Your body breaks down the protein that you eat into parts called amino acids. Your body then uses those amino acids to make other proteins that it needs to function.

PKU is a form of hyperphenylalaninemia. Hyperphenylalaninemia results in high levels of an amino acid called phenylalanine in the body.

Babies with PKU are unable to make an enzyme called phenylalanine hydroxylase (PAH) correctly. This enzyme helps change phenylalanine into another amino acid called tyrosine.

PKU can be more or less severe depending on how well PAH can break down phenylalanine. When the body does not make PAH correctly, but it can still process some phenylalanine, this results in a condition called non-PKU hyperphenylalaninemia. When PAH is missing or damaged, phenylalanine is not broken down and starts to build up in the body. This more severe condition is known as PKU. 

High levels of phenylalanine can damage the body, including the brain. If untreated, this damage leads to the signs and symptoms of the condition. However, with early diagnosis and treatment, children with PKU can have near-normal health and development.

Newborn Screening and Follow-Up

Condition Details

Treatment and Management

It is important to talk to your health care provider about which treatment(s) are best for your baby. The goal of treatment is to prevent the health problems caused by this condition.

Treatments may include the following:

  • Restricted diet low in foods that contain phenylalanine
  • Special phenylalanine-free formula
  • Medications that contain a substance called BH4 that helps the body process phenylalanine

Children who receive early and ongoing treatment for PKU can have healthy growth and development.

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