Given the increasing global burden of dementia, elucidating the relationship between thyroid disorders, as potentially modifiable risk factor of cognitive impairment was the main goal of this review.

Despite decades of research on the relation between thyroid diseases and cognition, the nature of this relationship remains elusive. Increasing prevalence of cognitive impairment and thyroid dysfunction has been consistently observed with aging. Also, there seems to be an association between thyroid disorders and cognitive decline.

We have summarized the current literature examining the relationship between thyroid dysregulation and cognition/ behavior. We present the available imaging and pathological findings related to structural and functional brain changes related to thyroid hormonal dysregulation.

Based on the available evidence, higher levels of thyroid hormones even within the normal range are associated with cognitive and behavioral changes and a detectable structural change in the brain. Hypothyroidism has also been shown to be associated with vascular pathologies of the brain. In addition, because free T4 (fT4) level remains stable with aging but the normal TSH level is age-dependent, it is reasonable to use fT4 for screening and monitoring treatment of thyroid hormonal dysregulation instead of TSH.

Presence of anti-thyroid Ab is another independent risk factor for cognitive and structural changes.

A long-term prospective study on healthy men and women in two separate groups might help clarify the pattern and temporal order of the changes in thyroid hormone (TH), TSH and autoantibodies with aging. The baseline information of participants will help establish possible genetic or environmental risk factors making certain part of the population more susceptible to thyroid dysregulation and its cognitive/behavioral effects.