Thyroid function and cognitive health: rethinking the relationship between "suboptimal" TSH/FT4 levels and cognitive...

Area of Science:

• Endocrinology
• Neurology
• Geriatrics

Background:

• Controversial hypotheses suggest suboptimal thyroid function may increase dementia risk.
• Suboptimal thyroid function is defined as higher-normal TSH (2.0-4.5 μIU/mL) or low-normal FT4.

Purpose of the Study:

• To investigate the association between suboptimal thyroid function and cognitive impairment in older adults.
• To evaluate the validity of claims linking thyroid function to cognitive decline.

Main Methods:

• Analysis of data from the Canadian Comprehensive Assessment of Neurodegeneration and Dementia cohort (ages 50-91).
• Thyroid function categorized as optimal (TSH < 2.0 μIU/mL or FT4 ≥ 16.74 pmol/L) or suboptimal.
• Included TSH data from 688 participants and FT4 data from 476 participants.

Main Results:

• No significant association was found between suboptimal thyroid function (TSH or FT4) and cognitive impairment.
• True hypothyroidism was rare, with 98.6% of participants with elevated TSH having normal FT4 levels.
• Chi-squared analyses yielded non-significant p-values for both TSH (p=0.67) and FT4 (p=0.96).

Conclusions:

• Findings do not support the hypothesis that higher-normal TSH or low-normal FT4 contribute to cognitive impairment.
• Caution is advised against unproven thyroid supplementation for cognitive health.
• The clinical justification for redefining TSH cutoffs is questioned, challenging functional medicine approaches.