Gtp Cyclohydrolase I Deficiency With Hyperphenylalaninemia Cn305333
A metabolic condition causing elevated phenylalanine levels due to impaired BH4 production. Requires lifelong dietary restriction and amino acid supplementation with regular monitoring.
Key facts at a glance
- ICD-10
- E88.9
- ICD-11
- 5A00.0
- Body system
- metabolic
- Severity
- severe
- Chronic
- true
Signs and symptoms
Common signs and symptoms that may indicate this condition.
Warning signs - seek immediate care
Seizures
Fatigue or weight changes
(metabolic)Significant symptoms that substantially affect daily functioning.
Frequency: Common at all stages
Intellectual disability
(neurological)Microcephaly
(neurological)Spasticity
(musculoskeletal)Diagnosis and tests
How this condition is diagnosed and what test results mean.
Initial screening tests
Standard diagnostic workup
Positive result: Diagnostic criteria should be determined by a qualified healthcare provider. [Content pending medical review.]
Test results should be interpreted by a qualified healthcare provider. [Content pending medical review.]
Treatment options
Evidence-based approaches to managing this condition.
First-line treatments
BH4 (Sapropterin) Supplementation
Synthetic cofactor replacement therapy that helps metabolize phenylalanine in patients with residual enzyme activity. Response varies based on genotype.
Drug therapy and clinical monitoring
Active treatment with regular monitoring by a healthcare provider is recommended.
Phenylalanine-Restricted Diet
Strict limitation of phenylalanine intake through specialized medical foods and careful food selection to maintain safe blood phenylalanine levels.
Additional treatment options
Medical Formula Supplementation
Provision of tyrosine-enriched amino acid formulas to ensure adequate nutrition when phenylalanine is restricted.
dietaryRegular Metabolic Monitoring
Frequent blood phenylalanine level testing and neurodevelopmental assessments to guide treatment adjustments.
monitoringNeurodevelopmental Support Services
Early intervention services to address motor delays and support optimal neurological development in affected children.
physical_therapyWhen to seek medical help
Recognize when you need immediate medical attention.
See a doctor within 24 hours
Moderate to severe symptoms that may indicate worsening of this condition. Prompt medical evaluation is recommended.
Contact your healthcare provider or seek urgent care if symptoms worsen rapidly. Go to the emergency department for life-threatening symptoms.
Frequently asked questions
Common questions about this topic.
What is Gtp Cyclohydrolase I Deficiency With Hyperphenylalaninemia Cn305333?
Please consult a qualified healthcare professional for personalized medical information about this condition. [Content pending medical review.]
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This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 25, 2026. · Read our editorial policy