Association of Thyroid Dysfunction with Cognitive Function:An Individual Participant Data Analysis

Importance: In clinical guidelines, overt and subclinical thyroid dysfunction are mentioned as causal and treatable factors for cognitive decline. However, the scientific literature on these associations shows inconsistent findings. Objective: To assess cross-sectional and longitudinal associations of baseline thyroid dysfunction with cognitive function and dementia. Design, Setting, and Participants: This multicohort individual participant data analysis assessed 114267 person-years (median, 1.7-11.3 years) of follow-up for cognitive function and 525222 person-years (median, 3.8-15.3 years) for dementia between 1989 and 2017. Analyses on cognitive function included 21 cohorts comprising 38144 participants. Analyses on dementia included eight cohorts with a total of 2033 cases with dementia and 44573 controls. Data analysis was performed from December 2016 to January 2021. Exposures: Thyroid function was classified as overt hyperthyroidism, subclinical hyperthyroidism, euthyroidism, subclinical hypothyroidism, and overt hypothyroidism based on uniform thyrotropin cutoff values and study-specific free thyroxine values. Main Outcomes and Measures: The primary outcome was global cognitive function, mostly measured using the Mini-Mental State Examination. Executive function, memory, and dementia were secondary outcomes. Analyses were first performed at study level using multivariable linear regression and multivariable Cox regression, respectively. The studies were combined with restricted maximum likelihood meta-analysis. To overcome the use of different scales, results were transformed to standardized mean differences. For incident dementia, hazard ratios were calculated. Results: Among 74565 total participants, 66567 (89.3%) participants had normal thyroid function, 577 (0.8%) had overt hyperthyroidism, 2557 (3.4%) had subclinical hyperthyroidism, 4167 (5.6%) had subclinical hypothyroidism, and 697 (0.9%) had overt hypothyroidism. The study-specific median age at b