Overview
Tetrahydrobiopterin-responsive phenylketonuria (BH4-responsive PKU) is a form of phenylketonuria (PKU) in which patients respond to treatment with a substance called tetrahydrobiopterin (BH4), also known as sapropterin. PKU is a metabolic condition where the body cannot properly break down an amino acid called phenylalanine (Phe), which is found in most protein-containing foods. In classic PKU, the enzyme phenylalanine hydroxylase (PAH) does not work well enough, causing phenylalanine to build up in the blood. High levels of phenylalanine can damage the brain and lead to intellectual disability, behavioral problems, and seizures if left untreated. In BH4-responsive PKU, the PAH enzyme still has some residual activity, and giving BH4 as a medication helps the enzyme work better. This means that patients with this form of PKU can often tolerate more protein in their diet compared to those with classic PKU who do not respond to BH4. Not all people with PKU respond to BH4 — typically those with milder mutations in the PAH gene are more likely to benefit. The main treatment is a combination of dietary phenylalanine restriction and BH4 supplementation (sapropterin dihydrochloride, sold as Kuvan). With early diagnosis through newborn screening and proper treatment, most individuals with BH4-responsive PKU can achieve good phenylalanine control and have normal or near-normal intellectual development. Regular blood monitoring of phenylalanine levels is essential throughout life.
Key symptoms:
High phenylalanine levels in the blood detected on newborn screeningIntellectual disability if untreatedBehavioral problems such as hyperactivity or anxietySeizures if untreatedLighter skin, hair, and eye color than family membersMusty or mousy body odorEczema or skin rashesDifficulty concentrating or learning problemsMood changes or irritabilityDelayed development if untreatedTremors or movement problems in poorly controlled cases
Autosomal recessive
Passed on when both parents carry the same gene change; often skips generations
Neonatal
Begins at or shortly after birth (first 4 weeks)
Treatments
No FDA-approved treatments are currently listed for Tetrahydrobiopterin-responsive phenylketonuria.
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Specialists
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Treatment Centers
8 centersBaylor College of Medicine Rare Disease Center ↗
Baylor College of Medicine
📍 Houston, TX
🏥 NORDStanford Medicine Rare Disease Center ↗
Stanford Medicine
📍 Stanford, CA
🔬 UDNNIH Clinical Center Undiagnosed Diseases Program ↗
National Institutes of Health
📍 Bethesda, MD
🔬 UDNUCLA UDN Clinical Site ↗
UCLA Health
📍 Los Angeles, CA
🔬 UDNBaylor College of Medicine UDN Clinical Site ↗
Baylor College of Medicine
📍 Houston, TX
🔬 UDNHarvard/MGH UDN Clinical Site ↗
Massachusetts General Hospital
📍 Boston, MA
🏥 NORDMayo Clinic Center for Individualized Medicine ↗
Mayo Clinic
📍 Rochester, MN
👤 Mayo Clinic Center for Individualized Medicine
🏥 NORDUCLA Rare Disease Day Program ↗
UCLA Health
📍 Los Angeles, CA
Travel Grants
No travel grants are currently matched to Tetrahydrobiopterin-responsive phenylketonuria.
Community
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Caregiver Resources
NORD Caregiver Resources
Support, advocacy, and financial assistance for caregivers of rare disease patients.
Mental Health Support
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Family & Caregiver Grants
Financial assistance programs specifically for caregivers of rare disease patients.
Social Security Disability
Learn how rare disease patients may qualify for SSDI/SSI benefits.
Questions for your doctor
Bring these to your next appointment
- Q1.What is my (or my child's) specific PAH gene mutation, and how does it affect BH4 responsiveness?,What phenylalanine level should we aim for, and how often should we check blood levels?,How much natural protein can be included in the diet while on sapropterin therapy?,What happens if phenylalanine levels go above the target range temporarily?,Are there any new treatments or clinical trials we should know about?,How should we manage phenylalanine levels during illness or times of stress?,What support is available for the emotional and psychological aspects of living with PKU?
Common questions about Tetrahydrobiopterin-responsive phenylketonuria
What is Tetrahydrobiopterin-responsive phenylketonuria?
Tetrahydrobiopterin-responsive phenylketonuria (BH4-responsive PKU) is a form of phenylketonuria (PKU) in which patients respond to treatment with a substance called tetrahydrobiopterin (BH4), also known as sapropterin. PKU is a metabolic condition where the body cannot properly break down an amino acid called phenylalanine (Phe), which is found in most protein-containing foods. In classic PKU, the enzyme phenylalanine hydroxylase (PAH) does not work well enough, causing phenylalanine to build up in the blood. High levels of phenylalanine can damage the brain and lead to intellectual disabilit
How is Tetrahydrobiopterin-responsive phenylketonuria inherited?
Tetrahydrobiopterin-responsive phenylketonuria follows a autosomal recessive inheritance pattern. Genetic counseling can help families understand recurrence risk and testing options.
At what age does Tetrahydrobiopterin-responsive phenylketonuria typically begin?
Typical onset of Tetrahydrobiopterin-responsive phenylketonuria is neonatal. Age of onset can vary across affected individuals.