Phenylketonuria (PKU)

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Phenylketonuria (PKU)

Topic Overview

What is phenylketonuria (PKU)?

Phenylketonuria (PKU) is a rare genetic disorder in which the body cannot break down an amino acid called phenylalanine, which is a part of protein. A baby with PKU needs treatment to start soon after birth. That way, phenylalanine (say "fehn-uhl-A-luh-neen") won't build up in the blood and brain. A buildup can lead to intellectual disability and central nervous system problems. But starting treatment soon after birth can prevent all or most of these problems.

What causes PKU?

PKU is passed down through families. To have the disease, a baby must get the gene from both parents. If a baby gets the gene from only one parent, he or she is a carrier of the PKU gene but does not have the disease.

What are the symptoms?

Symptoms usually start within a few months after birth, after phenylalanine has built up in the baby's body from the protein in formula or breast milk.

Early symptoms of PKU in a baby include:

  • A musty odour to the skin, hair, and urine.
  • Skin problems.
  • Losing weight from vomiting and diarrhea.
  • Acting fussy.
  • Being sensitive to light.

If PKU treatment is not started early enough, the child may have growth problems, developmental delays, seizures, and severe intellectual disability.

How is PKU diagnosed?

All babies born in the United States and Canada are tested for PKU within a few days after birth. The test may be repeated within the first week or two after birth.

Finding and starting treatment for PKU early usually can prevent brain damage and other long-term problems.

How is it treated?

The main treatment for PKU is a lifelong reduced-protein diet. Your baby’s doctor can help you choose the right formula for your baby. You may be able to feed your baby some breast milk, but talk to your baby’s doctor first. When your child gets older, a registered dietitian can help you choose the right foods and recipes.

People who have PKU need regular blood tests to check phenylalanine levels.

If you or your child has PKU, it’s normal to feel a wide range of emotions. You may want to join a support group. Your doctor also can help.

Can PKU be prevented?

If you have a child with PKU, it may be helpful to get genetic counselling if you are thinking about having another child. If you have a family history of PKU, talk with your doctor about genetic testing.

If you have PKU and you want to have a baby, you must carefully control your phenylalanine levels. Babies born to mothers who have high levels of phenylalanine during pregnancy are at risk for:

  • Severe problems with cognitive development.
  • Growth problems.
  • Heart problems present from birth (congenital heart disease).
  • Having a very small head (microcephaly).

Frequently Asked Questions

Learning about phenylketonuria (PKU):

Being diagnosed:

Getting treatment:


Symptoms of phenylketonuria (PKU) usually develop within a few months after birth, after phenylalanine has built up in a baby's system from consuming the protein in formula or breast milk. Before birth, the mother's body filters out the excess phenylalanine for the baby (fetus).

Early symptoms of PKU, which occur in more than half of babies with this condition, include:

  • A musty odour to the skin, hair, and urine.
  • Vomiting and diarrhea, leading to weight loss.
  • Irritability.
  • Skin problems, such as dry skin, or itchy skin rashes (eczema).
  • Sensitivity to light (photosensitivity).

If phenylalanine builds up in the brain tissue, it affects mental skills and the central nervous system. Symptoms can become severe by about 8 weeks of age and may include:

Finding and starting treatment for PKU soon enough can prevent a buildup of phenylalanine.

PKU also affects the synthesis of melanin, which provides pigment (colour) to the skin, eyes, and hair. Most people with PKU have blond hair, fair skin, and blue eyes.1

Examinations and Tests

Periodic blood tests to check phenylalanine levels are recommended for babies and pregnant women with phenylketonuria (PKU). Babies may get tested as often as once a week for the first year and then once or twice a month throughout childhood. Pregnant women with PKU should be tested at least once a week.

PKU is an autosomal recessive disease, which means it will only develop if a person inherits the gene from both parents. If the PKU gene is inherited from only one parent, the person will be a carrier of the gene for PKU but will not develop the disease. The risk of passing on an autosomal recessive disease depends on your family medical history. Ask your doctor about a genetic test if you are interested in confirming whether you carry the gene for PKU.

Early detection

Within a few days of birth, all babies in the United States and Canada are tested for PKU. The test for phenylketonuria uses two or three drops of blood taken from your baby's heel to measure phenylalanine levels. Phenylalanine blood levels are higher in babies with PKU.

Most doctors will take the blood sample after your baby has been fed, 24 to 48 hours after birth.4 It can take a few days for phenylalanine to build up in a newborn's body, so testing before 24 hours can give untrue results (false negatives).

Treatment Overview

The main treatment for phenylketonuria (PKU) consists of permanently reducing the amount of protein in the diet.

Protein contains phenylalanine, an amino acid that is essential for normal growth and development. People with PKU lack an enzyme to properly metabolize phenylalanine. Very high levels of phenylalanine in the blood can lead to irreversible brain and nervous system damage.

Soon after being diagnosed, babies with PKU start on a special formula without phenylalanine. You may also be able to feed your baby breast milk, but talk to your baby's doctor first. During your baby's first year, blood tests to check phenylalanine levels are usually done weekly.

Although any brain or nervous system damage that develops is irreversible, problems are less likely to occur if a PKU diet starts by 3 weeks of age. Your baby will most likely grow and develop normally when given formula without phenylalanine. In rare cases, children may have learning or behaviour problems (such as slower language development, hyperactivity, anxiety, and poor concentration) despite early treatment.

A reduced-protein diet should be consistently followed throughout life. As your child grows, the diet is individualized and adjusted according to his or her specific needs. Hunger levels, growth, development, behaviour, illness, food preferences, and phenylalanine blood levels are all taken into consideration.

It is important to have regular blood tests to measure phenylalanine levels. From ages 1 to 12, testing is usually done twice a month. After age 12 and throughout adulthood, testing once a month is suggested.2 High levels of phenylalanine in teens and adults negatively affect IQ (intelligent quotient) and cognitive functions such as awareness, knowledge, thinking, learning, and judgment.

Maternal PKU

The American Academy of Pediatrics and other expert groups have recommendations for managing PKU in women who are or who may become pregnant. Generally, women who have PKU and want to have a baby need to follow the PKU diet and get enough nutrients to support a growing fetus.

It is best to adopt the PKU diet before becoming pregnant. Intellectual disability can occur in as many as half of the children whose mothers have uncontrolled PKU during most of their pregnancy.5, 6 High levels of phenylalanine in the mother during pregnancy can also cause babies to be born with abnormally small heads (microcephaly), growth problems, and congenital heart disease.

Women with PKU who are considering becoming pregnant should also have periodic blood tests, as advised by their doctor, to check phenylalanine levels. Low levels (below 6 mg/dL) of phenylalanine should be reached at least 3 months before conception. Recommended levels are 2 mg/dL to 6 mg/dL.3 After a woman is pregnant, she should be tested at least once a week until the baby is born.

Home Treatment

Home treatment for phenylketonuria (PKU) focuses on managing phenylalanine levels throughout life.

Follow the dietary guidelines. Learn about the condition and explain its effects to your child. Stress the importance of staying on the PKU diet, and involve your child in selecting appropriate foods. It is a good idea to seek suggestions from a registered dietitian about types of foods, recipes, and how to ensure your child's nutritional needs are being met. Adults with PKU will also benefit from this information. Be aware that foods low in phenylalanine often cost more.

Work with your doctor. Set up a schedule for checking blood phenylalanine levels. Periodic testing is needed throughout life. The frequency depends on your age and severity of enzyme deficiency. It's also important for children who have PKU to get regular dental care, because protein substitutes can sometimes wear down the teeth.

Manage maternal PKU. If you are a woman with PKU who is considering pregnancy, follow the American Academy of Pediatrics (AAP) recommendations. High levels of phenylalanine in the mother could cause the baby to be born with an intellectual disability or with other health problems, such as an unusually small head (microcephaly). Following the PKU diet both before and during pregnancy will help keep your baby (fetus) healthy. Women with PKU should be tested for PKU levels twice a week during pregnancy.

Seek genetic testing. If you have a family history of PKU, talk with your doctor about a genetic test. If you have a child with PKU, it may be helpful to seek genetic counselling before attempting another pregnancy.

Join a support group. If you or your child has PKU, you may want to join a support group or talk with families who are dealing with similar issues. Remember that it is normal to have feelings of anger, depression, guilt, and frustration when living with PKU or when caring for someone else with the condition. Don't be afraid to talk with your doctor about such feelings. It may also be helpful to encourage your child to participate in a support group in which he or she can express personal feelings.

Other Places To Get Help


Canadian PKU and Allied Disorders
77 Cottingham Street
Toronto, ON M4V 1B9
Phone: 1-877-226-7581 toll-free
Web Address:

Canadian PKU and Allied Disorders provides accurate news, information, and support to families and professionals dealing with phenylketonuria and similar, rare, inherited metabolic disorders.

Canadian Organization for Rare Disorders
151 Bloor Street West
Suite 600
Toronto, ON  M5S 1S4
Phone: 1-877-302-7273 toll-free
(416) 969-7464
Web Address:

The Canadian Organization for Rare Disorders (CORD) provides information on over 6,000 rare conditions. Through a network system, CORD links together individuals and families with the same rare disorder.

Children's PKU Network
3790 Via De La Valle
Suite 120
Del Mar, CA  92014
Phone: 1-800-377-6677
(858) 509-0767
Fax: (858) 509-0768
Web Address:

The Children's PKU Network is a nonprofit organization that offers networking services, resources, a hotline, and family support.

Genetics Home Reference, U.S. National Library of Medicine
8600 Rockville Pike
Bethesda, MD  20894
Phone: 1-888-FIND-NLM (1-888-346-3656)
Fax: (301) 402-1384
TDD: 1-800-735-2258
Web Address:

The Genetics Home Reference provides information on hundreds of genetic conditions. The website has many tools for learning about human genetics and the way genetic changes can cause disease. It also has links to additional resources for people who have genetic conditions and for their families.



  1. Ropper AH, Samuels MA (2009). Inherited metabolic diseases of early childhood section of Inherited metabolic diseases of the nervous system. In Adams and Victor's Principles of Neurology, 9th ed., pp. 922–926. New York: McGraw-Hill.
  2. National Institutes of Health (2000). Phenylketonuria (PKU): Screening and management. NIH Consensus Statement, 17(3): 1–33.
  3. Kaye CI, and the Committee on Genetics, American Academy of Pediatrics (2006). Phenylketonuria section of Newborn screening fact sheets: Technical report. Pediatrics, 118(3): e956–e957.
  4. Rezvani I (2007). Phenylalanine section of Defects in metabolism of amino acids. In RM Kliegman et al., eds., Nelson Textbook of Pediatrics, 18th ed., chap. 85, pp. 529–532. Philadelphia: Saunders Elsevier.
  5. Waisbren SE, Azen C (2003). Cognitive and behavioral development in maternal phenylketonuria offspring. Pediatrics, 112(6, Part 2): 1544–1547.
  6. Wappner RS (2006). Disorders of phenylalanine and tyrosine metabolism section of Disorders of amino acid and organic acid metabolism. In JA McMillan et al., eds., Oski's Pediatrics: Principles and Practice, 4th ed., chap. 385, pp. 2153–2157. Philadelphia: Lippincott Williams and Wilkins.

Other Works Consulted

  • Aminoff M (2009). Metabolic disorders section of Neurologic disorders. In RK Creasy et al., eds., Creasy and Resnik's Maternal-Fetal Medicine, 6th ed., pp. 1101–1102. Philadelphia: Saunders Elsevier.
  • Durkin MS, et al. (2008). Childhood cognitive disability. In RB Wallace et al., eds., Wallace/Maxcy-Rosenau-Last Public Health and Preventive Medicine, 15th ed., pp. 1173–1183. New York: McGraw-Hill.
  • Sapropterin (Kuvan) for phenylketonuria (2008). Medical Letter on Drugs and Therapeutics, 50(1287): 43–44.
  • Thomas JA, Van Hove JLK (2009). Phenylketonuria and the hyperphenylalaninemias section of Inborn errors of metabolism. In WW Hay Jr et al., eds., Current Diagnosis and Treatment: Pediatrics, 19th ed., pp. 966–967. New York: McGraw-Hill.


By Healthwise Staff
Primary Medical Reviewer Susan C. Kim, MD - Pediatrics
Primary Medical Reviewer Anne C. Poinier, MD - Internal Medicine
Specialist Medical Reviewer Thomas Emmett Francoeur, MD, MDCM, CSPQ, FRCPC - Pediatrics
Last Revised June 8, 2010

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