Frequently Asked Questions About PKU

Get answers to some of the most common questions about PKU.

Facts and information

Q: What is PKU?

A: Phenylketonuria (PKU), or phenylalanine hydroxylase (PAH) deficiency, is an inherited disease that causes an amino acid known as phenylalanine to build up in the body in people with PKU. Proteins found in foods such as chicken, meat, eggs, dairy, nuts, grains, and legumes are broken down into amino acids by enzymes in the body. The body then uses these amino acids to grow and repair body tissue, build muscle, provide energy, and make other important substances. Phenylalanine, also called Phe, is one of the many amino acids used by the body for these normal functions. In people without PKU, a liver enzyme called phenylalanine hydroxylase, or PAH, converts Phe to another amino acid known as tyrosine. Tyrosine then travels from the liver through the bloodstream to the brain, where it plays a critical role in the development of brain neurotransmitters, which are important chemicals that affect thinking, mood, and behavior. In someone with PKU, the body can’t process Phe because the PAH enzyme is either missing or not working properly, which causes blood Phe levels to rise. High and unstable blood Phe levels can negatively affect how people with PKU think, feel, and behave.

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Q: What is the difference between PKU and PAH deficiency?

A: There is no difference between PKU and PAH deficiency.

Recent medical guidelines have started referring to PKU as PAH deficiency, which better represents the spectrum of disease severity.

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Q: What are the different levels of PKU severity?

A: People with PKU can have different levels of PAH deficiency, resulting in differences in Phe levels and symptoms. Different PKU ranges are classified as classic PKU, moderate or variant PKU, mild PKU, and hyperphenylalaninemia. Over the years, people with classic and moderate PKU can have impaired neurological function if Phe levels are not well controlled.

Classic PKU is the most severe level of PKU, because the level of enzyme deficiency is greater. Left untreated, a person with classic PKU will have very high levels of Phe in the blood, >1200 µmol/L (>20 mg/dL).

A person with moderate or variant PKU, who has more residual enzyme activity, will have lower blood Phe levels than a person with classic PKU, if both are eating the same diet. Levels are generally somewhere in the range of 900-1200 µmol/L (15-20 mg/dL).

A person with mild PAH deficiency will have lower levels of Phe in their blood (600-900 µmol/L or 10-15 mg/dL), because they have more residual (working) PAH enzyme activity.

A person with Phe levels that are slightly above levels of someone without PAH deficiency has hyperphenylalaninemia, with Phe levels of 120-600 µmol/L (2-10 mg/dL).

Please note that ranges may vary depending on treatment facility standards.

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Q: How do you get PKU?

A: PKU is an inherited disorder, meaning you can’t “catch it.” It is something you are born with. Usually, for a person to have PKU, both the mother and father must have passed on the mutated PAH gene. As a parent, you can be a carrier with only one copy of the mutated gene, but not have PKU yourself.

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Q: Did I cause my child's PKU?

A: PKU is inherited, so parents are not to blame for their child's PKU. There is nothing that either parent did prior to or during the pregnancy to cause PKU. This is important to remember. PKU is a genetic condition. Every human being carries mutated genes. If both the mother and father carry one copy of the same disease­-causing mutated gene, they won’t have PKU themselves, but each of their children has a 25% chance of having PKU. If one parent has PKU, but the other parent doesn't carry it, any child they have is also unlikely to have PKU.

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Q. Is PKU serious?

A: Yes. PKU is serious. If it is not managed from birth and continuously throughout life, PKU can cause health and mental problems, including severe intellectual disability and neurological impairments, as well as behavioral, emotional, and social problems. Later in life, it can cause many other problems, such as lower IQ, depression, anxiety, and feeling “foggy” or being unable to focus. But there is good news. If diagnosed and managed early and continuously throughout life, people with PKU can be healthy and able to lead normal lives.

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Q: What are the symptoms of PKU?

A: Symptoms of PKU can vary from mild to severe. The symptoms of PKU are caused by high blood Phe levels. If untreated in childhood, high blood Phe levels can cause severe intellectual disability. Even in babies and children who are treated early and continuously, high blood Phe levels may cause symptoms such as being cranky or fretful, inability to focus, and anxiety.

In teens and adults, when blood Phe levels are too high, problems can include:

  • Not being able to focus or pay attention
  • Feeling anxious
  • Feeling depressed
  • Slower thinking and responding
  • Feeling "foggy"
  • Bad moods, being cranky or irritable
  • Difficulty remembering things
  • Lower IQ
When blood Phe levels are low, Phe will not accumulate in the brain.

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Q: What can happen if someone with PKU goes off the low-Phe diet?

A: If a child goes off the diet very early in life (from birth to the elementary school years), he or she may develop physical or mental impairments that may or may not be reversible. This is why it is so important to stay on a low­-Phe diet.

For a person who goes off the diet later in life, some of the symptoms may include:

  • Not being able to focus or pay attention
  • Feeling anxious
  • Feeling depressed
  • Slower thinking and responding
  • Feeling "foggy"
  • Bad moods, being cranky or irritable
  • Lower IQ
Learn how high blood Phe can affect the brain.
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Q: Is PKU curable?

A: To date, there is no cure for PKU or PAH deficiency. Click here to learn more about treatment options.

Treatment guidelines recommend that all people with PKU, except those with two null mutations in trans, try prescription medication to determine if it can help lower blood Phe levels.1 Talk to your healthcare professional to find out if KUVAN (sapropterin dihydrochloride) could be an option for you or your child. With careful early and lifelong treatment management, people with PKU can live healthier and more productive lives.

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Indication

KUVAN® (sapropterin dihydrochloride) Tablets for Oral Use and Powder for Oral Solution are approved to reduce blood Phe levels in people with a certain type of Phenylketonuria (PKU). KUVAN is to be used with a Phe-restricted diet.

Important Safety Information

It is not possible to know if KUVAN will work for you without a trial of the medicine.

Your doctor will check your blood Phe levels when you start taking KUVAN to see if the medicine is working.

Starting KUVAN does not eliminate the need for ongoing dietary management. Any change to your diet may impact your blood Phe level. Follow your doctor’s instructions carefully. Your doctor and dietitian will continue to monitor your diet and blood Phe levels throughout your treatment with KUVAN to make sure your blood Phe levels are not too high or too low. If you have a fever, or if you are sick, your Phe level may go up. Tell your doctor and dietitian as soon as possible so they can make any necessary changes to your treatment.

Children younger than 7 years old treated with KUVAN doses of 20 mg/kg per day are at an increased risk for low levels of blood Phe compared with children 7 years and older. Frequent blood monitoring is recommended in this population to ensure that blood Phe levels do not fall too low.

Tell your doctor if you have ever had liver or kidney problems, have poor nutrition or have a loss of appetite, are pregnant or plan to become pregnant, or are breastfeeding or plan to breastfeed.

KUVAN is a prescription medicine and should not be taken by people who are allergic to any of its ingredients. KUVAN and other medicines may interact with each other. Tell your doctor about all the medicines you take, including prescription and over-the-counter medicines, vitamins, herbal and dietary supplements.

If you forget to take your dose of KUVAN, take it as soon as you remember that day. Do not take 2 doses in a day. If you take too much KUVAN, call your doctor for advice.

The most common side effects reported when using KUVAN are headache, runny nose and nasal congestion, sore throat, diarrhea, vomiting, and cough. Additional adverse reactions reported in connection with worldwide marketing include sore throat, heartburn or pain in the esophagus, inflammation of the lining of the stomach, indigestion, stomach pain, and nausea. These are not all the possible side effects seen with KUVAN. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

KUVAN can cause serious side effects, including:

  • Severe allergic reactions. Stop taking KUVAN and get medical help right away if you develop any of these symptoms of a severe allergic reaction:
    • Wheezing or trouble breathing
    • Nausea
    • Flushing
    • Lightheadedness or fainting
    • Coughing
    • Rash
  • Inflammation of the lining of the stomach (gastritis). Gastritis can happen with KUVAN and may be severe. Call your doctor right away if you have any:
    • Severe upper stomach-area discomfort or pain
    • Blood in your vomit or stool
    • Nausea and vomiting
    • Black, tarry stools
  • Too much or constant activity (hyperactivity) can happen with KUVAN. Tell your doctor if you have any signs of hyperactivity, including fidgeting, moving around or talking too much.

For more information, call BioMarin RareConnectionsTM at 1-877-MY-KUVAN (1-877-695-8826). Please read the full Patient Information by clicking here.

Sources: Blau N, van Spronsen FJ, Levy HL. Phenylketonuria. Lancet. 2010;376(9750):1417-1427.
Vockley J, Andersson HC, Antshel KM, et al; for the American College of Medical Genetics and Genomics Therapeutic Committee. Phenylalanine hydroxylase deficiency: diagnosis and management guideline. Genet Med. 2014;16(2):188-200.