Skip to Content
Home > Health Information > Health Library > Phenylketonuria (PKU)
Phenylketonuria (PKU) is a rare genetic disorder in which the body cannot break down an amino acid called phenylalanine (say "fehn-uhl-AL-uh-neen"), which is a part of protein. This substance is found in breast milk, many types of baby formula, and most foods, especially those with a lot of protein, such as meat, eggs, and dairy products. If PKU is not treated, phenylalanine can build up in the blood and lead to intellectual disability and problems with the central nervous system (brain and spinal cord).
The good news is that early treatment can prevent all or most problems. Babies born with PKU need to start treatment with special formula soon after birth.
PKU is more common in whites and Native Americans than in blacks, Hispanics, and Asians.
PKU is passed down through families. For a baby to have the disease, he or she must get (inherit) the PKU gene from both parents. The father and mother may not have PKU or even know that PKU runs in their families. 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.
If you have a child with PKU and are thinking about having another baby, you may want to get genetic counseling. If you have a family history of PKU, talk with your doctor about genetic testing if you want to find out whether you carry the gene.
If PKU is not found and treated soon after birth, symptoms usually start to appear within a few months after birth. (It takes time for the phenylalanine to build up in the baby's body.)
Early symptoms of PKU in a baby may include:
Without early treatment, the child may have growth problems, developmental delays, seizures, and severe intellectual disability.
Screening is recommended for all newborns within a few days after birth.footnote 1 If the PKU screening test shows that your baby has a phenylalanine problem, the doctor will do further testing to check whether your baby has PKU.
Finding and starting treatment for PKU early usually can prevent brain damage and other long-term problems.
The main treatment for PKU is a lifelong reduced-protein diet. Problems are less likely to occur if your baby starts a PKU diet by age 3 weeks. Your baby's doctor can help you choose a formula that doesn't contain phenylalanine. You may be able to feed your baby some breast milk, but talk to your baby's doctor first. As your child gets older, a registered dietitian can help you choose the right foods and recipes.
The medicine sapropterin (Kuvan) may help lower phenylalanine levels in some children who have PKU. More studies are needed to find out the health effects from long-term use. With or without the medicine, your child still needs to follow a PKU diet.
Your child will need regular blood tests. He or she may get tested as often as once or more a week for the first year and then once or twice a month throughout childhood.
It's normal to feel a wide range of emotions when your child has an illness like PKU. You may want to join a support group. Your doctor can also help.
People who have PKU need to follow a reduced-protein diet throughout their lives. For a few people, treatment with sapropterin (Kuvan) may help when used with a PKU diet. Talk to your doctor about the pros and cons of this medicine.
You'll need regular blood tests to check your phenylalanine level. If the level builds up, it can affect your IQ and your ability to learn, think, and understand.
Any woman with PKU who is planning to have a baby needs to be very careful to control her phenylalanine levels. Babies born to mothers who have high levels of phenylalanine are at risk for having a very small head, intellectual disability, growth problems, and heart problems. Having regular blood tests and following the PKU diet before and during pregnancy can help protect the baby.
U.S. Preventive Services Task Force (2008). Screening for phenylketonuria (PKU). Available online: http://www.uspreventiveservicestaskforce.org/uspstf/uspsspku.htm.
Other Works Consulted
Aminoff M (2009). Neurologic disorders. In RK Creasy et al., eds., Creasy and Resnik's Maternal-Fetal Medicine, 6th ed., pp. 1089–1112. Philadelphia: Saunders.
Committee on Genetics, American Academy of Pediatrics (2008). Maternal phenylketonuria. Pediatrics, 122(2): 445–449.
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.
Feillet F, et al. (2010). Challenges and pitfalls in the management of phenylketonuria. Pediatrics, 126(2): 333–341.
Kaye CI (2006, reaffirmed 2011). Newborn screening fact sheets. Pediatrics, 118(3): e934–e963.
Poustie VJ, Wildgoose J (2010). Dietary interventions for phenylketonuria. Cochrane Database of Systematic Reviews (1).
Sapropterin (Kuvan) for phenylketonuria (2008). Medical Letter on Drugs and Therapeutics, 50(1287): 43–44.
Schwahn BC (2011). Hyperphenylalaninemias. In CD Rudolph et al., eds., Rudolph's Pediatrics, 22nd ed., pp. 561–563. New York: McGraw-Hill.
Somaraju UR, Merrin M (2012). Sapropterin dihydrochloride for phenylketonuria. Cochrane Database of Systematic Reviews (12).
Thomas J, pet al. (2014). Inborn errors of metabolism. In WW Hay Jr et al., eds., Current Diagnosis and Treatment: Pediatrics, 22nd ed., pp. 1106–1133. New York: McGraw-Hill.
Current as of: December 12, 2018
Author: Healthwise StaffMedical Review: John Pope, MD, MPH - PediatricsKathleen Romito, MD - Family Medicine
Current as of:
December 12, 2018
Medical Review:John Pope, MD, MPH - Pediatrics & Kathleen Romito, MD - Family Medicine
To learn more about Healthwise, visit Healthwise.org.
© 1995-2019 Healthwise, Incorporated. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.
Our interactive Decision Points guide you through making key health decisions by combining medical information with your personal information.
You'll find Decision Points to help you answer questions about:
Get started learning more about your health!
Our Interactive Tools can help you make smart decisions for a healthier life. You'll find personal calculators and tools for health and fitness, lifestyle checkups, and pregnancy.
Feeling under the weather?
Use our interactive symptom checker to evaluate your symptoms and determine appropriate action or treatment.
See important information from Billings Clinic on Non-discrimination and Interpreter Services.
Billings Clinic is proud to be an equal opportunity/affirmative action (EOE/AA) e-verify employer.