Thyrotropin level and cognitive performance: Baseline results from the ELSA-Brasil Study.

BACKGROUND AND AIMS: The role of subtle thyroid alterations, such as subclinical thyroid disease and low/high serum thyrotropin (TSH) within the normal range, on cognitive decline is controversial. The aim of this study was to evaluate the association of serum TSH and subclinical thyroid dysfunction with performance on cognitive tests in a large sample of Brazilian middle-aged adults without overt thyroid disease.

METHODS: In this cross-sectional analysis of the Brazilian Longitudinal Study of Adult Health, we excluded individuals aged 65 years and older, with overt thyroid dysfunction, prevalent stroke, in use of medications that affect thyroid function or that indicate neurologic diseases, and from Asian or indigenous ethnicity. Thyroid status was assessed by serum TSH and free thyroxine (only when the TSH was altered). Individuals were divided according to TSH tertiles and classified according to thyroid function as euthyroidism, subclinical hypothyroidism, or subclinical hyperthyroidism. Cognition was evaluated using delayed word recall test, semantic verbal fluency test, and trail making test version B. The associations of cognitive tests performance with TSH tertiles (using the middle tertile as reference) and thyroid function were investigated using linear regression models, adjusted for an extensive set of possible confounders (sociodemographic characteristics, cardiovascular risk factors, and depression).

RESULTS: The mean age of the 10,362 participants was 49.5±7.4years, 52.3% women. After adjustment for confounders, the first TSH tertile was associate with worse performance on the trail making test (β=-0.05, 95% CI=-0.09; -0.01, p=0.017). When restricting the analysis to the 9769 individuals with TSH within the normal range, the association between TSH and performance on the trail making test remained significant (β=-0.05, 95% CI=-0.09; -0.01, p=0.020) on multiple linear regression. Subclinical thyroid disease was not associated with performance on cognitive tests.

CONCLUSION: Low TSH is associated with poorer performance on an executive function test in middle-aged adults without overt thyroid dysfunction.