Tetrahydrobiopterin deficiency consists of four different defects: GTPCH deficiency or guanosine triphosphate cyclohydrolase I deficiency- with autosomal dominant (Segawa syndrome) and autosomal recessive forms, dihydropteridine reductase (DHPR) deficiency, PCD deficiency i.e. pterin-4-alpha-carbinolamine dehydratase deficiency and 6-pyruvoyl tetrahydropterin synthase deficiency or PTPS deficiency. All these pathological entities lead to high levels of phenylalanine and impaired production of serotonin, catecholamines and nitric oxide, leading to neurologic, psychiatric and cardiovascular dysfunction, as well as abnormal embryonic development.
Tetrahydrobiopterin deficiency may cause a wide variety of symptoms in various individuals, depending on disease severity and transient or permanent character. Disease signs also depend on the type of the defect, with PCD deficiency being less severe and often causing transient motor delay. In general, newborns may appear normal, except for those with severe PTPS deficiency who have low birth weight and are frequently premature. However, all affected newborns may present with microcephaly and unsatisfactory ponderal curve due to inefficient sucking and swallowing. An increased risk for type 2 diabetes during infancy has also been described.
More severe forms are characterized by seizures, trunk hypotonia, and limb hypertonia, both considered to be extrapyramidal neurologic signs  or "floppy baby" appearance and difficulty in swallowing. Motion abnormalities consist of opisthotonus, chorea, bradykinesia, and athetosis. These may be accompanied by intellectual and psychomotor retardation, that may improve with treatment . Older children may be red-headed and experience gait abnormalities, dystonia , as well as upward, downward or convergent oculogyric crises that can be accompanied by malaise, lacrimation, and excessive blinking.
Associated symptoms include hypersalivation, irritability, and hyperthermia. The general health status may have diurnal fluctuations. Clinical as well as neurological status deteriorate as the child grows older  . The physician performing the examination should keep in mind that the central nervous system, pulmonary and cardiac microcirculation are at risk in these conditions, making late detection and delayed treatment result in poor prognosis .
Clinical evaluation and patient history raise tetrahydrobiopterin deficiency suspicion, but this must be confirmed using blood workup. However, the diagnosis can be suggested during newborn screening in the presence of elevated phenylalanine levels. In this case, differential diagnosis between tetrahydrobiopterin deficiency and phenylketonuria is needed. Furthermore, phenylalanine can be initially normal and diagnosis may be suggested by progressive and unexplainable neurological impairment. The next step in the workup plans is represented by detection of biopterin and neopterin in the urine and dried blood spots . Several other pterins may also be present in the urine: isoxanthopterin, primapterin, monapterin, and pterin. In GTPCH deficiency, both biopterin and neopterin are low, in PTPS deficiency, neopterin is high and biopterin is low, while in DHPR deficiency both may be elevated or neopterin may be normal. In PCD deficiency, primapterin may also be detected.
Tetrahydrobiopterin loading test  can exclude phenylketonuria and consists of the reduction of high phenylalanine levels after tetrahydrobiopterin administration. Other diagnostic tests include DHPR level evaluation using enzyme assay or measurement of folates, pterins or neurotransmitter metabolites like homovanillic acid or 5-hydroxyindoleacetic acid in the cerebrospinal fluid.
In uncertain cases, molecular genetic testing may be necessary, despite being expensive and not widely available.
Guthrie cards can be used to measure DHPR activity in the red blood cells. Other enzymes (PTPS or GTPCH) activities van also be determined in the white blood cells or fibroblasts. Also, the physician should keep in mind that prolactin levels can be high in DHPR deficiency.