A rarer form of hyperphenylalaninemia is tetrahydrobiopterin deficiency, which occurs when the PAH enzyme is normal, and a defect is found in the biosynthesis or recycling of the cofactor tetrahydrobiopterin (BH4). BH4 is necessary for proper activity of the enzyme PAH, and this coenzyme can be supplemented as treatment. Those who suffer from this form of hyperphenylalaninemia may have a deficiency of tyrosine (which is created from phenylalanine by PAH), in which case treatment is supplementation of tyrosine to account for this deficiency.