To compare differences in clinical data and thyroid function between patients with Parkinson's disease with dementia (PDD) and those without dementia (nPDD), and to explore correlation between thyroid hormone levels and different cognitive domains in PD patients. Methods: Clinical and thyroid function data were collected from 38 PDD and 50 nPDD patients. All participants completed the assessments including UPDRS-Ⅲ score, H-Y staging, MMSE, MoCA (Beijing version), ADL, HAMA, and HAMD scales. The levels of free triiodothyronine (FT3), free thyroxine (FT4), and thyroid hormone (TSH) were measured in all participants. The differences in general clinical data and thyroid hormone levels between the two groups were compared, and the correlation between thyroid hormone levels and clinical data and different cognitive domains was analyzed. Results: Compared to the nPDD group, patients in the PDD group had higher UPDRS-Ⅲ scores, higher equivalent daily dose of Levodopa, shorter education duration, and higher ADL, HAMA, and HAMD scores (P<0.05). There were no significant differences in TSH, FT3, and FT4 levels between PDD and nPDD patients (P>0.05). In PD patients, TSH levels were positively correlated with gender (r=0.294, P<0.01), and negatively correlated with HAMA scores (r=－0.246, P<0.05); FT3 levels were negatively correlated with age (r=－ 0.293, P<0.01) and gender (r=－ 0.320, P<0.01), and positively correlated with MoCA total score (r= 0.391, P<0.01), visuospatial and executive function (r=0.630, P<0.01), language (r=0.222, P<0.05), and delayed memory (r=0.307, P<0.01). After adjusting for confounding variables, multiple regression analysis showed that FT3 levels were positively correlated with visuospatial and executive function (P<0.01). Conclusion: PDD patients had shorter educational duration, more severe symptoms, higher equivalent daily dose of levodopa, poorer daily life abilities, and higher levels of anxiety and depression compared to nPDD patients. There were no significant differences in TSH, FT3, and FT4 levels between PDD and nPDD patients. Lower FT3 levels in PD patients were associated with worse cognitive function, particularly in visuospatial and executive function.