TH and tyrosine hydroxylase deficiency: In particular, variants in TH, the enzyme that converts L-tyrosine (L-Tyr) to the DA precursor L-3,4-dihydroxyphenylalanine (L-Dopa)23, result in TH deficiency (THD), with phenotypes ranging from L-Dopa-responsive infantile parkinsonism with dystonia to severe encephalopathy with neonatal onset24.