Subclinical thyroid dysfunction ups risk of dementia, cognitive decline
Subclinical hyperthyroidism with suppressed thyroid stimulating hormone (TSH) is linked to a greater risk of dementia and cognitive decline, according to data from the Health, Aging and Body Composition study.
The population-based prospective cohort study included 2,558 adults aged 70 to 79 years, among whom 85 percent were euthyroid (TSH 0.45 to 4.49 mIU/L), 2 percent had subclinical hyperthyroidism with mildly decreased TSH (TSH 0.10 to 0.44 mIU/L), 1 percent had subclinical hyperthyroidism with suppressed TSH (TSH <0.10 mIU/L with normal free thyroxine [FT4]) and 12 percent had subclinical hypothyroidism (TSH 4.50 to 19.99 mIU/L with normal FT4). None of the patients had dementia at baseline.
Incident dementia (based on the Modified Mini-Mental State [3MS] examination, hospital records and dementia drugs) was the primary study outcome. Changes in 3MS were also evaluated.
In the cohort, 22 percent developed dementia over 9 years. The risk of dementia was higher in patients with subclinical hyperthyroidism with suppressed TSH than in euthyroid individuals (hazard ratio [HR], 2.38; 95 percent CI, 1.13 to 5.04). Such association was not observed for mildly decreased TSH (HR, 0.79; 0.45 to 1.38) or subclinical hypothyroidism (HR, 0.91; 0.70 to 1.19).
Furthermore, individuals with subclinical hyperthyroidism with suppressed TSH showed a greater decline in 3MS (-3.89; -7.62 to -0.15).
The relationship between hyperthyroid dysfunction and dementia may be explained by several pathophysiological pathways, researchers said.
“First, dementia may be caused by an increase in neuronal necrosis and oxidative stress associated with both hyperthyroidism and Alzheimer’s disease (AD). Second, a genetic susceptibility may exist, as some thyroid hormones target genes are involved in neurogenesis. Third, dementia could be caused by lower choline in the brain, as described in AD,” they added.
Additional prospective studies with larger sample size or participant data analysis with information on different types of dementia and with a larger number of participants with subclinical hyperthyroidism are needed to confirm the present data and ascertain more specific associations, according to researchers.