Abstract: Objective To investigate the correlation between serum thyroid hormone levels and mild cognitive impairment (MCI) in patients with type 2 diabetes mellitus (T2DM). Methods A total of 341 T2DM patients who were admitted to General Hospital of Eastern Theater Command from June 2016 to December 2024 were selected. According to the Montreal Cognitive Assessment (MoCA) scores, the patients were divided into two groups: non-MCI (MoCA≥26, n=190) and MCI (MoCA<26, n=151). Their general information was collected, while biochemical indicators and thyroid function indicators, such as triiodothyronine (T3), free triiodothyronine (FT3), thyroxine (T4), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) were measured. Multivariate logistic regression analysis was performed to determine the independent risk factors for MCI in T2DM patients. Results Compared with the non-MCI group, the MCI group showed remarkable increases in age, T2DM duration, higher fasting plasma glucose (FPG) levels, and decreases in fasting C-peptide (FCP) levels (P<0.05). Furthermore, the T3 and FT3 levels in the MCI group were significantly lower than in the non-MCI group (P<0.05). After adjusting for confounding factors such as age, T2DM duration, FPG, and FCP, multivariate logistic regression analysis indicated that for every 1 μg/L increase in T3 levels, the risk of MCI in T2DM patients decreased by 66.7% (OR=0.333, 95% CI 0.225-0.494, P<0.01). Conclusions T3 is an independent protective factor for MCI in T2DM patients, with potential clinical predictive value.