<b><i>Background:</i></b> Alterations in thyroid hormone (TH) levels may be related to the pathogenesis of mild cognitive impairment (MCI) and dementia. Cognitive deficits are common in Parkinson's disease (PD) patients. The aim of this study was to investigate whether variations within the normal ranges of thyroid function are related to cognitive function in early PD without dementia. <b><i>Methods:</i></b> Eighty-four euthyroid patients with early PD underwent evaluation of their thyroid status, including measures of thyroid-stimulating hormone (TSH), total triiodothyronine (tT₃) and free thyroxine (fT₄), and comprehensive neuropsychological tests. <b><i>Results:</i></b> The 46 patients of the PD-MCI group did not differ in the serum levels of TH compared to the 38 patients of the PD-normal cognition group. fT₄ levels were inversely associated with the Mini-Mental State Examination (MMSE) score and neuropsychological tests of attention and visuospatial and executive function. TSH and tT₃ levels were not related to cognitive performances. After controlling for demographic and clinical variables, multiple regression analyses indicated statistically significant associations between fT₄ concentrations and MMSE score and neuropsychological tests of executive function. <b><i>Conclusions:</i></b> This study supports a relationship between the thyroid status and cognitive function in euthyroid early PD patients, with higher concentrations of fT₄ being associated with a poor performance of executive function.