Low levels of free triiodothyronine are associated with risk of cognitive impairment in older euthyroid adults.

Accumulated evidence showed that thyroid diseases induced cognitive decline. However, the relationship between thyroid hormones (THs) and cognition in older euthyroid people is still unclear. Our study aimed to estimate the association between THs within the euthyroid range and cognition in community-dwelling older adults in China. Data were extracted from a cohort study on the health status of rural older adults from the Guizhou province in China (HSRO). Serum thyroid-stimulating hormone (TSH), free thyroxine (FT), and free triiodothyronine (FT) were measured using the electrochemiluminescence immunoassay. Cognitive function was evaluated by the Mini-Mental State Examination (MMSE). Linear regression and a binary logistic regression model were used to explore the relationship between THs and cognition in euthyroidism (TSH level of 0.27 ~ 4.20mIU/L). A total of 957 euthyroidism individuals were included in this study, with a mean (SD) age of 71.34 (6.35) years. In individuals with euthyroidism, serum TSH and FT levels were positively associated with cognition (TSH:β = 0.06, 95% CI  0.01 ~ 0.11, P = 0.03; FT:β = 0.07, 95% CI  0.01 ~ 0.12, P = 0.01); and serum FT and TSH levels were significantly associated with cognitive domains (P < 0.05). Further, euthyroid individuals in the lowest serum FT(OR = 1.96; 95% CI 1.27 ~ 3.03) quartile had a twofold increased risk of cognitive impairment compared to those in the highest quartile after adjusting for potential confounding factors. These findings suggested that low levels of FT could be an independent risk factor for cognitive impairment in older euthyroid adults. Additionally, a positive linear association exists between serum FT levels and cognitive domains (such as immediate memory, language, and attention). Further studies are needed to determine the underlying mechanisms and the community significance of these findings.