Thyroid function and cognitive health: rethinking the relationship between « suboptimal » TSH/FT4 levels and cognitive decline

Abstract

Introduction: Controversial hypotheses suggest that « suboptimal » thyroid function – defined as higher-normal thyroid-stimulating hormone (TSH) (2.0 to 4.5 μIU/mL) or low-normal free thyroxine (FT4) – may increase risk of cognitive decline and dementia in older patients.

Methods: We examined this claim using data from the Canadian Comprehensive Assessment of Neurodegeneration and Dementia cohort (ages 50 to 91; cognitive statuses from unimpaired to dementia, rarer subtypes of dementia excluded). Thyroid function was categorized as « optimal » (TSH < 2.0 μIU/mL or FT4 ≥ 16.74 pmol/L) or « suboptimal. » Six hundred eighty-eight participants had TSH data, and 476 participants had FT4 data.

Results: Chi-squared analyses showed no significant association between « suboptimal » thyroid function and cognitive impairment (TSH: χ² = 0.178, p = 0.67; FT4: χ² = 0.0027, p = 0.96). True hypothyroidism was rare; 98.6% of those with elevated TSH had normal FT4.

Discussion: These findings do not support claims that higher-normal TSH or low-normal FT4 contribute to cognitive impairment. Caution is warranted against advocating unproven thyroid supplementation for cognitive health.