Tetrahydrobiopterin (BH4) deficiency, also known as hyperphenylalaninemia caused by a defect in biopterin metabolism, is a rare inherited disorder that increases the amount of the amino acid phenylalanine to harmful levels in the bloodstream. (Amino acids are the building blocks of proteins.) Although phenylketonuria (PKU) also increases phenylalanine levels and can have similar symptoms, the two disorders are caused by different genes and are treated differently. Tetrahydrobiopterin deficiency is a subtype of hyperphenylalaninemia. As in PKU, high levels of phenylalanine are present from infancy in people with BH4 deficiency. The resulting signs and symptoms range from mild (such as temporary low muscle tone) to severe. Severe symptoms include mental retardation, progressive problems with development, movement disorders, difficulty swallowing, an inability to control body temperature, seizures, and behavioral problems.