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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: = 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. HIGHLIGHTS: No link was found between "suboptimal" thyroid function and cognitive impairment. Higher-normal TSH levels do not indicate dementia risk or hypothyroidism. Among participants with elevated TSH, 98.6% had normal FT4 levels. Redefining TSH cutoffs to 2.0 μIU/mL lacks clinical justification. Findings question the functional medicine approach of thyroid supplementation for cognitive health.
Dori et al. (Mon,) studied this question.