Catalyzes the conversion of L-tyrosine to L- dihydroxyphenylalanine (L-Dopa), the rate-limiting step in the biosynthesis of catecholamines, dopamine, noradrenaline, and adrenaline. Uses tetrahydrobiopterin and molecular oxygen to convert tyrosine to L-Dopa (PubMed:15287903, PubMed:1680128, PubMed:17391063, PubMed:24753243, PubMed:34922205, PubMed:8528210, Ref.18). In addition to tyrosine, is able to catalyze the hydroxylation of phenylalanine and tryptophan with lower specificity (By similarity). Positively regulates the regression of retinal hyaloid vessels during postnatal development (By similarity). {ECO:0000250|UniProtKB:P04177, ECO:0000250|UniProtKB:P24529, ECO:0000269|PubMed:15287903, ECO:0000269|PubMed:1680128, ECO:0000269|PubMed:17391063, ECO:0000269|PubMed:24753243, ECO:0000269|PubMed:34922205, ECO:0000269|PubMed:8528210, ECO:0000269|Ref.18}. [Isoform 5]: Lacks catalytic activity. . [Isoform 6]: Lacks catalytic activity. . This is the function of Ensembl gene identifier ENSG00000180176 (TH, tyrosine hydroxylase).