The Parkinson's disease (PD) risk gene GTP cyclohydrolase 1 (GCH1) catalyzes the ratelimiting step in tetrahydrobiopterin (BH4) synthesis, an essential cofactor in the synthesis of monoaminergic neurotransmitters. To investigate the mechanisms by which GCH1 deficiency may contribute to PD, we generated a loss of function zebrafish gch1 mutant (gch1 -/-), using CRISPR/Cas technology. gch1 -/zebrafish develop marked monoaminergic neurotransmitter deficiencies by 5 dpf, movement deficits by 8 dpf and lethality by 12 dpf.Tyrosine hydroxylase protein levels were markedly reduced without loss of ascending dopaminergic (DAergic) neurons. L-Dopa treatment of gch1 -/larvae improved survival without ameliorating the motor phenotype. RNAseq of gch1 -/larval brain tissue identified highly upregulated transcripts involved in innate immune response. Subsequent experiments provided morphological and functional evidence of microglial activation in gch1 -/-. The results of our study suggest that GCH1 deficiency may unmask early, subclinical parkinsonism and only indirectly contribute to neuronal cell death via immune-mediated mechanisms. Our work highlights the importance of functional validation for GWAS risk factors and further emphasises the important role of inflammation in the pathogenesis of PD.