Background: Thyroid hormones are essential for normal development and function of the central nervous system. Thyroid dysfunction is associated with many neuropsychiatric disorders mainly cognitive impairment. Results: We found a close correlation between thyroid status and cognitive dysfunction. Serum FT3 levels decreased, whereas the serum thyroid-stimulating hormone (TSH) level increased, with the decline in cognitive functions. Furthermore, the TSH level showed a negative correlation with the Mini-Mental State Examination (MMSE) scores. We suggested that thyroid function was associated with cognitive impairments induced by subcortical ischemic vascular dementia (SIVD). Conclusion: We found that thyroid dysfunction especially subclinical hypothyroidism is associated with cognitive impairment. Dementia increases more with more increase in TSH, and the MMSE score decreases with the increase of age.