Abstract
Introduction: The role of subtle thyroid alterations on cognitive decline is controversial. We aimed to investigate the association of thyrotropin (TSH) and free thyroxine (FT4) with baseline performance on cognitive tests and with cognitive decline in middle-aged and older adults without overt thyroid disease. Methods: This is a longitudinal analysis of the Brazilian Longitudinal Study of Adult Health baseline and second wave, after 4 years of follow-up. We included participants aged ≥ 55 years without over thyroid disease, prevalent stroke and use of drugs that could affect thyroid function and cognition. TSH and FT4 were assessed at baseline. Cognition was evaluated at baseline and at the second wave using delayed word recall test (DWR), semantic verbal fluency test (SVF), and trail making test version B (TMT). Baseline and longitudinal associations of TSH tertiles and FT4 tertiles with cognitive performance were investigated with generalized estimating equations, adjusted for sociodemographic characteristics, lifestyle, cardiovascular risk factors, and depression. Results: The baseline mean age of the 4675 participants was 62.4 ± 5.8 years, 52.3% women (2445 out of 4675). At baseline, TSH levels were not associated with cognitive performance in any test, although the highest FT4 tertile was associated with poorer performance on DWR (β = -0.087, 95% CI = -0.155; -0.019) and SVF (β = -0.076, 95%CI = -0.143; -0.010) after adjustment. Additionally, the lowest FT4 tertile was associated with poorer performance on SVF (β = -0.090, 95%CI = -0.152; -0.028). Cognitive performance did not change after 4 years of follow-up and there was no effect of time on the association of thyroid hormone levels with cognitive performance. Conclusion: At baseline, FT4 levels were associated with worse cognitive performance in a relatively young sample. Neither baseline FT4 nor TSH were associated with cognitive decline after 4 years.
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