TH is a key enzyme in the production of dopamine, and although there are no definitive studies demonstrating the changes of TH, a decrease in VMAT2 mRNA levels in PD platelets (Sala et al., 2010) and a decrease in the amount of dopamine stored in vesicles leads to a decrease in intracellular dopamine concentration, which may inhibit TH activity. Here, TH is linked to Parkinson disease.