GTPCH1 Deficiency (Biopterin Deficiency) GTP-cyclohydrolase I deficiency, is an autosomal recessive disorder of tetrahydrobiopterin metabolism. Tetrahydrobipterin plays a role as a cofactor in Phenylalanine, L-Dopa and Serotonin metabolism. Thus deficiencies are mostly associated with hyperphenylalaninemia, and deficiency of Dopamine and Serotonine. Unless diagnosed by newborn screening via high blood phenylalanine levels, patients develop neurological problems within the first year of life, including progressive developmental delay, seizures, microcephaly, and Parkinsonian dystonia. Episodes of irritability, lethargy, poor temperature control and other vegetative dysfunctions are common in severe cases. Apart from the autosomal recessive form, there is also an autosomal dominant form of GTP-cyclohydrolase I deficiency which is characterised by L-dopa responsive dystonia (Segawa Syndrome). Dystonia starts mostly in the legs, can be asymmetric and often occurs with diurnal fluctuation. Although patients with typical Segawa Syndrome usually have normal cognitive functions, there is a continuum of severity between Segawa Syndrome and autosomal recessive GTP-cyclohydrolase deficiency. Blood Phenylalanine is not elevated and thus patients are not identified via a positive neonatal screening test for phenylketonuria.