Thyroid Function and Cognitive Decline: A Narrative Review

Objective As the population of older adults in the United States continues to rise, understanding modifiable risk factors that contribute to cognitive decline and dementia becomes increasingly important. This narrative review summarizes existing literature on the association between thyroid function in the euthyroid range, hypothyroidism and hyperthyroidism, and cognitive outcomes in older adults. Methods A comprehensive literature search of the PubMed and Ovid/Medline databases was conducted. Randomized controlled trials, systematic reviews, meta-analyses and observational studies published in English between January 2000 and December 2023 were included. Results Overall, existing studies yielded conflicting results, failing to delineate a concrete relationship between thyroid function and cognitive outcomes and/or dementia in older adults. There may be a possible association between higher TSH in the reference range and lower risk of incident dementia, which may be more pronounced in women. Majority of studies elucidated a possible association between low TSH and incident dementia, with suggestion that duration of hyperthyroidism may contribute to increasing dementia risk. Even though evidence on the association of hypothyroidism and cognitive decline are disparate, current data do not support treatment of subclinical hypothyroidism to improve cognitive outcomes in older adults. Conclusion Despite numerous studies, there is no conclusive evidence that supports a direct relationship between hyperthyroidism or hypothyroidism and cognitive decline. Study limitations include a heterogeneity in study designs, measurement methodologies, and cognitive assessment tools. Future research is needed to better delineate whether an association exists and whether treatment of thyroid dysfunction ameliorates cognitive impairment.