Background: The relationship between clinical as well as subclinical thyroid disease and cardiovascular disease has been reported. Epidemiological evidence is accumulating that vascular risk factors increase the risk of AD. We sought to investigate the relationship between thyroid function and dementia. Methods: We examined patients who were admitted to geriatric ward of one psychiatric hospital in Kyong-Gi. The control group was individuals who underwent check up at a health promotion center of one university hospital in Daejeon. Results: The number of patients with dementia was 110. In the dementia group, there were 12 people with subclinical hyperthyroidism, 6 with subclinical hypothyroidism, and 2 with apparent hypothyroidism. After controlling for education, hypertension, diabetes and total cholesterol, multiple logistic regression yielded the odds ratio for dementia to be significantly higher in the group with low levels of thyroid stimulating hormone compared to the normal group (OR 4.00, 95% confidence interval 1.04~15.39). Conclusion: We found a significant relationship between dementia and subclinical hyperthyroidism.