Introduction: Changes in the levels of serum components can help in the early and differential diagnosis of Parkinson’s disease (PD). The lesion in the dopaminergic (DAergic) system is the main pathophysiological mechanism underlying PD. This system is closely related to the hypothalamic-pituitary-thyroid axis. Here, we examined the impact of PD onset, progress, and severity in rats on the serum levels of thyroid hormones (THs). Methods: The neurotoxin 6-OHDA was injected into the medial forebrain bundle. Behavioral tests were carried out for eight weeks after the toxin to assess the severity of PD and its progress. Blood was collected before the toxin and in the third and eighth weeks afterward. THs levels were determined using specific ELISA kits. Results: Our findings show that the THs levels changed significantly after the toxin. The levels of T3 and T4 decreased slightly in the third week but remarkably increased in the eighth week. The decrease in the third week depended on the severity of the PD and was observed only in the rats with severe behavioral symptoms. On the other hand, the increase in the eighth week occurred in all 6-OHDA-received rats with severe or mild behavioral symptoms. Conclusion: Our data indicate that serum levels of THs may decrease and increase in PD. At the onset of the disease, the levels may decrease if DAergic neuronal death is severe. In the progress phase of PD, THs levels may increase independent of the severity of the disease.