Clinical review: The thyroid in mind: cognitive function and low thyrotropin in older people

Several studies have reported an association between low serum TSH, or subclinical hyperthyroidism (SH), and dementia, but little emphasis has been placed on this field because not all studies have demonstrated the same association. We performed a detailed systematic review to assess the evidence available to support the association between these two conditions.

We performed a systematic search through the PubMed, Embase (1996 to 2012 wk 4), Cochrane Library, and Medline (1996 to January wk 4, 2012) electronic databases using key search terms encompassing subclinical hyperthyroidism, TSH, dementia, and cognitive impairment.

This review examines the 23 studies that provide information about the association between SH or lower serum TSH within the reference range and cognition. Fourteen of these studies, including several well-designed and well-powered cross-sectional and longitudinal analyses, have shown a consistent finding of an association between SH with cognitive impairment or dementia.

There is a substantial body of evidence to support the association between SH and cognitive impairment, but there is no clear mechanistic explanation for these associations. Nor is there an indication that antithyroid treatment might ameliorate dementia. Larger and more detailed prospective longitudinal or randomized controlled trials are required to inform these important questions.