Background: Thyroid hormones are essential for normal development and function of the central nervous system. 9 Thyroid dysfunction is associated with many neuropsychiatric disorders mainly cognitive impairment. 10 Results: We found a close correlation between thyroid status and cognitive dysfunction. Serum FT3 levels decreased, 11 whereas the serum thyroid-stimulating hormone (TSH) level increased, with the decline in cognitive functions. 12 Furthermore, the TSH level showed a negative correlation with the Mini-Mental State Examination (MMSE) scores. We 13 suggested that thyroid function was associated with cognitive impairments induced by subcortical ischemic vascular 14 dementia (SIVD). 15 Conclusion: We found that thyroid dysfunction especially subclinical hypothyroidism is associated with cognitive 16 impairment. Dementia increases more with more increase in TSH, and the MMSE score decreases with the increase of age.