Conditions Benign Hyperphenylalaninemia

Benign hyperphenylalaninemia (H-PHE) is a mild form of phenylketonuria. It is considered an amino acid condition because people with H-PHE have problems breaking down an amino acid, a building block of proteins, known as phenylalanine. Most people with this condition experience mild symptoms or no symptoms.

Condition Type

Amino Acid Disorders


The number of people affected with H-PHE is currently unknown. One estimate claims that H-PHE affects 15-75 babies out of every one million births. It is considered a rare condition.

Also known as

  • H-PHE
  • HPA
  • Hyper-PHE
  • Non-phenylaketonuria – mild type
  • Variant hyperphenylalaninemia
  • PKU variant
  • Hyperphenylalaninemia
  • Hyperphenylketonuria

Follow-Up Testing

Your baby’s doctor may ask you if your baby is showing any of the signs of H-PHE (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 benign hyperphenylalaninemia (H-PHE) is 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. 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 signs of H-PHE. Children with H-PHE have more phenylalanine in their bodies than is typical, but they have a lower amount of phenylalanine in their bodies than do children with the condition known as classic phenylketonuria. Measuring the amount of phenylalanine in your baby’s body can help doctors determine if your baby has a condition. Sometimes follow-up testing may include testing a very small sample of skin.

About Benign Hyperphenylalaninemia

Early Signs

Benign hyperphenylalaninemia (H-PHE) is a form of phenylketonuria (PKU). Different forms of PKU have varying severity of signs. Because H-PHE is a less severe type of PKU, babies with H-PHE typically do not show any signs.

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


Babies with benign hyperphenylalaninemia (H-PHE) usually do not require any treatment.

Some individuals with more severe forms of H-PHE may need to limit the amount of phenylalanine in their diet and drink a special medical formula that contains no phenylalanine. Phenylalanine is a substance found in many proteins and also in artificial sweeteners. Babies with H-PHE cannot break down phenylalanine as quickly as babies without H-PHE. Your baby’s doctor can help determine how much phenylalanine is safe for your baby.

Expected Outcomes

Babies with benign hyperphenylalaninemia (H-PHE) typically have no complications. They can have healthy growth and development.

However, some babies with H-PHE do have a small risk of brain damage without treatment. This is why newborn screening for H-PHE is important.


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 benign hyperphenylalaninemia (H-PHE), the enzyme phenylalanine hydroxylase (PAH) is not working correctly.

PAH’s job is to break down the amino acid phenylalanine. Babies with H-PHE make less PAH than babies without H-PHE. They can break down phenylalanine, but not as quickly as babies without H-PHE. If the body cannot break down phenylalanine quickly enough, phenylalanine can build up in the blood. Everyone has some phenylalanine in their blood, but high levels could be harmful. Babies with H-PHE have elevated levels of phenylalanine, but these levels are usually not dangerous.

H-PHE is an autosomal recessive genetic condition. This means that a child must inherit two copies of the non-working gene for H-PHE, 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 H-PHE 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 Benign Hyperphenylalaninemia

Support Services

Support groups can help connect families who have a child or other family member affected with benign hyperphenylalaninemia 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:

Accessing Care

Work with your baby’s health care provider to determine the next steps for your baby’s care. Your baby’s doctor may help you coordinate care with medical resources in your community, which may include a physician who specializes in metabolism or a dietician. Some children with benign hyperphenylalaninemia (H-PHE) 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 H-PHE 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 H-PHE, 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 Engineoffered by the American College of Medical Genetics and Genomics and the Find a Genetic Counselor tool on the National Society of Genetic Counselors (NSGC) website are two good places for you or your baby's health care provider to use to identify local specialists.

Families' Experiences

At this time we have not located a family story for this particular condition. If your family is affected by benign hyperphenylalaninemia (H-PHE) and you would like to share your story, please contact us so other families may benefit from your experience.

References & Sources

Visit the Screening, Technology And Research in Genetics (STAR-G) Project for more information on amino acid disorders

Visit Medscape for information on benign hyperphenylalaninemia

Visit the Newborn Screening Coding and Terminology Guide by the U.S. Library of Medicine for condition information

Visit MedicineNet for information on phenylketonuria

Visit MedlinePlus Genetics for more 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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