Cognitive impairment associated with low but still 'normal' TSH

Cognitive impairment can occur at any age with suboptimal thyroid function—neurons require thyroid stimulus as much as any other cells. A study just published JCEM (The Journal of Clinical Endocrinology & Metabolism) offers evidence that TSH (thyroid stimulating hormone) levels when low but still 'normal' are associated with cognitive impairment and dementia. The authors state:

"The association between subclinical hyperthyroidism and the risk of dementia has been validated in several studies. However, the effect of thyroid function within reference range on the risk of cognitive dysfunction including mild cognitive impairment (MCI) and dementia is still unclear...Our aim was to investigate the association between thyroid function and the risk of MCI and dementia in euthyroid elderly subjects."

313 participants who were euthyroid (normal thyroid) and not demented at the beginning of their study were followed for 5 years and evaluated for baseline thyroid function and cognitive impairment or dementia during the study period. They found a significant association between low-normal TSH levels and cognitive impairment:

"At baseline evaluation, 237 subjects were cognitively normal, and 76 subjects were MCI. Diagnoses of cognitive function in 259 subjects remained unchanged or improved during the study period (non-progression group), whereas 54 subjects showed progression of cognitive impairment to MCI or dementia (progression group). In the progression group, baseline serum TSH levels were lower than those in non-progression group. Baseline serum free-T4 levels were not significantly different between these two groups. The association between lower baseline serum TSH levels and the development of MCI or dementia was maintained after adjustment for conventional baseline risk factors."

Clinical note: Practitioners should bear in mind the various possible factors besides subclinical hyperthyroidism that can suppress TSH to low 'normal' levels that on their own contribute to cognitive impairment including chronic inflammation.The authors conclude:

"Lower serum TSH level within reference range was independently associated with the risk of cognitive impairment including MCI and dementia in elderly subjects."