Context:The association between subclinical hyperthyroidism and the risk of dementia has been validated in several studies. However, the effect of thyroid function within reference range on the risk of cognitive dysfunction including mild cognitive impairment (MCI) and dementia is still unclear.Objective:Our aim was to investigate the association between thyroid function and the risk of MCI and dementia in euthyroid elderly subjects.Design, setting, and participants:We conducted a population-based prospective study as a part of the Korean Longitudinal Study on Health and Aging (KLoSHA). 313 participants who were euthyroid and non-demented at baseline and completed cognitive function tests at 5-year-follow-up evaluation (mean age, 72.5 ± 6.9 yrs) were analyzed in the present study.Main outcome measure:Baseline thyroid function was compared according to the development of MCI or dementia during the study period. Binary logistic regression analysis was performed to investigate the independent association between thyroid function and cognitive impairment.Results:At baseline evaluation, 237 subjects were cognitively normal, and 76 subjects were MCI. Diagnoses of cognitive function in 259 subjects remained unchanged or improved during the study period (non-progression group), whereas 54 subjects showed progression of cognitive impairment to MCI or dementia (progression group). In the progression group, baseline serum TSH levels were lower than those in non-progression group. Baseline serum free-T4 levels were not significantly different between these two groups. The association between lower baseline serum TSH levels and the development of MCI or dementia was maintained after adjustment for conventional baseline risk factors.Conclusions:Lower serum TSH level within reference range was independently associated with the risk of cognitive impairment including MCI and dementia in elderly subjects.