Tetrahydrobiopterin deficiency is a rare condition that escalates the levels of various substances in the blood, including phenylalanine, an amino acid acquired through a diet. It is usually found in artificial sweeteners and all proteins. If tetrahydrobiopterin deficiency is left untreated, phenylalanine can develop to dangerous levels in the person’s body, causing severe health problems and intellectual disorder. 

This disease is common among newborn babies and is genetic in an autosomal recessive pattern. This means that two mutated genes are inherited from each parent. Moreover, the parent of a person with autosomal recessive disorder carries one mutated gene. However, they don’t usually manifest symptoms and signs of the condition. 

Additionally, tetrahydrobiopterin deficiency can be caused by a mutation in one of the various genes. These genes give an order for creating enzymes that help recycle and generate tetrahydrobiopterin in the body. If one of these enzymes malfunction properly due to gene mutation, there will be a lack of tetrahydrobiopterin to assist in processing phenylalanine. Consequently, phenylalanine may develop in other tissues and blood. And since brain nerve cells are oversensitive to phenylalanine, too much of this can cause brain damage. 


The symptoms of tetrahydrobiopterin deficiency include:

  • Movement disorders
  • Intellectual disability
  • Seizures
  • Difficulty in swallowing
  • Behavioral problems
  • Progressive problems with development
  • Incapacity to manage body temperature

Elevated levels of phenylalanine have existed from early childhood in people with untreated tetrahydrobiopterin deficiency. Also, newborns with this disease seem normal at birth, yet medical issues varying from mild to serious can become perceptible over time. 


The diagnosis of tetrahydrobiopterin deficiency depends upon recognition of the symptoms. A detailed patient history, tests, and clinical assessment will be needed to diagnose the condition. Moreover, the tests used to help in diagnosis may include urine tests, lumbar punctures, and blood tests

This condition is common among infants. Therefore, it is best to have an early diagnosis to determine the cause of the symptoms and give the proper treatment. 


The treatment of tetrahydrobiopterin deficiency focuses on controlling the symptoms and preventing long-term damage in the nervous system. This treatment may include:

  • Low phenylalanine diet
  • Neurotransmitters
  • Supplementation with tetrahydrobiopterin
  • And other medications to help ease the symptoms

If this condition is left untreated, it can cause severe health problems and intellectual disorder. So, visiting a doctor may be needed to prevent complications. 

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