Conditions Classic Phenylketonuria

Phenylketonuria (PKU) is a condition in which the body cannot break down one of the amino acids found in proteins. PKU is considered an amino acid condition because people with PKU cannot break down the amino acid called phenylalanine. If left untreated, PKU can cause brain damage or even death. However, if the condition is detected early and treatment is begun, individuals with PKU can lead healthy lives.

Phenylketonuria is a condition with multiple forms, each of which have different treatments and outcomes. Classic phenylketonuria is only one form of the condition. You can read about a different form of the condition, hyperphenylalanemia, here.

Condition Type

Amino Acid Disorders


In the United States, one in every 10,000 to 15,000 babies is affected by phenylketonuria (PKU). The occurrence of PKU varies among ethnic groups and regions. PKU is more common in individuals with Irish, northern European, Turkish, or Native American ancestry. It is less common in people of African, Japanese, or Ashkenazi Jewish backgrounds.

Also known as

  • PKU
  • Hyperphenylalaninemia – classic type
  • Phenylalanine hydroxylase deficiency
  • PAH deficiency
  • Phenylketonuria

Follow-Up Testing

Your baby’s doctor may ask you if your baby is showing any of the signs of PKU (see Early Signs below). If your baby has certain signs, your baby’s doctor may suggest starting immediate treatment.

If your baby’s newborn screening result for phenylketonuria (PKU) was out of the normal range, your baby’s doctor or the state screening program will contact you to arrange for your baby to have additional testing. It is important to remember that an out-of-range screening result does not necessarily mean that your child has the condition. An out-of-range result may occur because the initial blood sample was too small or the test was performed too early. However, as a few babies do have the condition, it is very important that you go to your follow-up appointment for a confirmatory test. Because the harmful effects of untreated PKU can occur soon after birth, follow-up testing must be completed as soon as possible to determine whether or not your baby has the condition.

Follow-up testing will involve checking your baby’s urine and blood samples for harmful levels of acids and toxins. Certain acids and toxins build up in the body when a child has an amino acid condition, so measuring the levels of these substances in your baby’s body can help doctors determine if your baby has a condition. High amounts phenylalanine in the blood might indicate that your baby has PKU.

About Classic Phenylketonuria

Early Signs

Different forms of phenylketonuria vary in their severity of signs. Classic phenylketonuria (PKU) is the most severe form. Babies with PKU usually seem healthy at birth. Signs of PKU begin to appear around six months of age.

Signs of classic PKU include:

  • Irritability
  • Seizures (epilepsy)
  • Dry, scaly skin (known as eczema)
  • “Musty” or "mouse-like” body odor
  • Pale hair and skin
  • Developmental delays

Many of these signs may occur when your baby eats foods that their body cannot break down. They can be triggered by long periods of time without eating, illnesses, and infections.

If your baby shows any of these signs, be sure to contact your baby’s doctor immediately.

If you are looking for the signs of hyperphenylalaninemia, another form of PKU, you’ll find them on the hyperphenylalaninemia page.


Dietary Treatment

Your baby will need to be on a restricted diet to avoid phenylalanine, a building block of proteins that individuals with phenylketonuria (PKU) cannot break down. Phenylalanine is found in all foods that contain protein and also in artificial sweeteners. The dietary treatment for PKU includes specific medical foods: a phenylalanine-free medical formula that is given regularly throughout the day, as well as foods modified to be low in protein. A registered dietician will help you plan a low-protein diet, that avoids high levels of phenylalanine, and gives your baby the nutrients he or she needs for healthy growth.

Supplements and Medication

Your baby’s doctor may prescribe a low protein formula. As your baby gets older, their doctor may prescribe a medication that contains BH4. BH4 is a substance naturally produced by the body, but your baby’s body might not make enough of it. Taking BH4 supplements may help break down the phenylalanine that builds up. Your baby’s doctor will need to write a prescription for these supplements. BH4 does not work in everyone with PKU. A trial period on the drug with evaluation by a physician is necessary.

Expected Outcomes

If phenylketonuria (PKU) is detected at birth, early treatment can prevent the signs of the condition mentioned in the Early Signs section. This is why it is so important to screen for PKU at birth.

If babies start treatment several weeks after birth, some signs of PKU can be avoided.

If treatment is started after six months of age, babies are at risk for severe intellectual disabilities.

It is important to treat PKU, even if treatment is started after noticing signs and symptoms, in order to help prevent permanent brain damage.


When we eat food, enzymes help break it down. Some enzymes break down protein into its building blocks, called amino acids. Other enzymes break down these amino acids. In classic phenylketonuria (PKU), the enzyme phenylalanine hydroxylase (PAH) is not working correctly.

PAH’s job is to break down the amino acid phenylalanine. Babies with PKU either do not make enough or make non-working PAH. When PAH does not work correctly, the body cannot break down phenylalanine and it builds up in the blood. Everyone has some phenylalanine in their blood, but high levels can be toxic.

PKU is an autosomal recessive genetic condition. This means that a child must inherit two copies of the non-working gene for PKU, one from each parent, in order to have the condition. The parents of a child with an autosomal recessive condition each carry one copy of the non-working gene, but they typically do not show signs and symptoms of the condition. While having a child with PKU is rare, when both parents are carriers, they can have more than one child with the condition. Learn more about autosomal recessive inheritance.

Support for Classic Phenylketonuria

Support Services

Support groups can help connect families who have a child or other family members affected with phenylketonuria (PKU) with a supportive community of people who have experience and expertise in living with the condition. These organizations offer resources for families, affected individuals, health care providers, and advocates:

The New England Genetics Collaborative created a website called "GEMSS: Genetics Education Materials for School Success." When your child reaches school age, you can provide teachers and school administration with this resource specifically for children with PKU.

Accessing Care

Work with your baby’s doctor to determine the next steps for your baby’s care. Your baby’s physician may help you coordinate care with a doctor who specializes in metabolism, a dietician who can help plan your child’s specialized diet, and other medical resources in your community. Some children with phenylketonuria (PKU) have developmental delays. If you think that your baby is not meeting their developmental milestones, ask your baby’s doctor about the next steps in accessing a developmental evaluation and care.

Because PKU is a genetic condition, you may want to talk with a genetics specialist. A genetic counselor or geneticist can help you understand the causes of the condition, discuss genetic testing for PKU, and understand what this diagnosis means for other family members and future pregnancies. Speak with your baby’s doctor about getting a referral. The Clinic Services Search Engine offered by the American College of Medical Genetics and Genomics (ACMG) and the Find a Genetic Counselor tool on the National Society of Genetic Counselors (NSGC) website are two good resources for you or your baby's health care provider to use to identify local specialists.

Families' Experiences

Kristin Vanags writes about her son Joseph who was diagnosed with phenylketonuria (PKU) through newborn screening. Currently at age three, Joseph is healthy and happy. His mom is growing more and more confident that her family will be able to manage the condition. Read Joseph’s story on the Georgia PKU Connect website.

Anna Parker was diagnosed with PKU through newborn screening. Now a healthy elementary school teacher, Anna considers herself blessed to have a treatable condition. Watch a video about her story here.

References & Sources

Visit the Screening, Technology And Research in Genetics (STAR-G) Project for more information on classic phenylketonuria

Visit MedlinePlus Genetics for more condition information

ACT Sheets

Healthcare professionals can learn more about confirmatory testing by reading the American College of Medical Genetics and Genomics' Algorithm for Diagnosis and ACT Sheet, a guide for follow-up after newborn screening. You can visit this page of the ACMG website here.

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