TH and mitochondrial disease: Other mitochondrial diseases are, however, more commonly associated with low HVA and 5-HIAA levels (García-Cazorla et al. 2007; Garcia-Cazorla et al. 2008a), although not as low as in primary neurotransmitter disorders (such as tyrosine hydroxylase or aromatic L-amino acid decarboxylase deficiency), where much lower CSF levels are usually reported (Ng et al. 2015).