TH and Parkinson disease: As would be expected, the mRNA expression level of tyrosine hydroxylase (TH), the rate limiting enzyme in monoamine synthesis, was disease-dependently reduced, in line with degeneration of dopaminergic neurons in the SNpc in PD (Fig. 2, early and late PD cases, 0.55 ± 0.25 and 0.22 ± 0.09 fold change from control respectively, p > .05 and p < .05).