Tetrahydrobiopterin Deficiency Overview Tetrahydrobiopterin (BH4) deficiencies is a general term for a group of disorders characterized by abnormalities in the creation (biosynthesis) or regeneration of tetrahydrobiopterin, a naturally occurring compound that acts as a cofactor. A cofactor is a non-protein substance in the body that enhances or is necessary for the proper function of certain enzymes. When tetrahydrobiopterin is deficient, the chemical balance within the body is upset. In most of these disorders, there are abnormally high levels of the amino acid phenylalanine (hyperphenylalaninemia). Amino acids such as phenylalanine are chemical building blocks of proteins and are essential for proper growth and development. Most of these disorders also cause abnormally low levels of neurotransmitters. Neurotransmitters are chemicals that modify, amplify or transmit nerve impulses from one nerve cell to another, enabling nerve cells to communicate. These chemical imbalances can ultima...