Subtle Thyroid Dysfunction Is Not Associated with Cognitive Decline: Results from the ELSA-Brasil.

Background: Subtle thyroid alterations have a controversial role in cognition.
Objective: We investigated the longitudinal association of baseline thyroid function, thyrotropin (TSH), and thyroxine (FT4) levels with cognitive performance after 4 years of follow-up in middle-aged and older adults without overt thyroid dysfunction.
Methods: We included 4,473 individuals, age≥55 years at the second study wave, without overt thyroid dysfunction at baseline. Individuals were divided according to thyroid function and TSH and FT4 tertiles. Cognition was assessed at baseline and after 4 years of follow-up by the word recall (DWR), semantic verbal fluency (SVF), and trail making (TMT) tests. The longitudinal association of thyroid function and TSH and FT4 tertiles with cognitive performance was investigated using generalized estimating equations adjusted for sociodemographic characteristics, lifestyle, cardiovascular risk factors and depression.
Results: There was no longitudinal association of thyroid function and TSH and FT4 baseline levels with performance on the cognitive tests. However, there was a baseline cross-sectional U-shaped association of FT4 tertiles with poorer performance in the SVF (first FT4 tertile: β= -0.11, 95% CI = -0.17; -0.04; third FT4 tertile: β= -0.10, 95% CI = -0.17; -0.04) and of the third FT4 tertile with poorer performance in the DWR (β= -0.09, 95% CI = -0.16; -0.02).
Conclusion: Thyroid function and hormone levels were not associated with cognitive decline during 4 years of follow-up in middle-aged and older adults without overt thyroid dysfunction. Future studies with longer follow-up could clarify the implications of subtle thyroid alterations in cognition.