BACKGROUND
Decreased brain function and dementia are important causes of disability among older people worldwide. While age is still the most critical risk factor, younger people can also have dementia (called early-onset dementia). It is estimated that 2-8% of all dementia cases are early-onset, and therefore early identification of potentially reversible risk factors responsible for decreased brain function and dementia is essential.

Thyroid disease has traditionally been felt to contribute to decline in brain functioning. As such, most clinical practice guidelines recommend that doctors order thyroid hormone blood tests when investigating their patients for dementia. However, the actual relationship between thyroid disease and brain function is still controversial. In fact, recent trials have found no evidence that treating various forms of hyperthyroidism or hypothyroidism will improve brain function. In the current study the authors try to answer whether or not thyroid problems are associated with decreased brain function and dementia.

THE FULL ARTICLE TITLE
van Vliet NA et al Thyroid Studies Collaboration 2021 Association of thyroid dysfunction with cognitive function: An individual participant data analysis. JAMA Intern Med. Epub 2021 Sep 7.

SUMMARY OF THE STUDY

The authors examined data from 74, 565 people participating in 23 different clinical trials whose thyroid hormone levels had been measured (57.5% women and 42.5% men).

Of these participants, 89.3% had normal thyroid function, 0.8% were hyperthyroid, 0.9% were hypothyroid, 3.4 % had subclinical hyperthyroidism (low TSH, normal FT4) and 5.6% had subclinical hypothyroidism (high TSH, normal FT4). In addition, 38,144 of these participants had provided data about their brain functioning and 2033 people were diagnosed with dementia.

They found that thyroid function, particularly subclinical hyperthyroidism and subclinical hypothyroidism, was not related to normal brain function, a decrease in brain function or risk of dementia. Similarly, there was no association between overt hypothyroidism and overt hyperthyroidism either, although the data was more limited for these two groups.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
The authors conclude that since they could find no statistical association between thyroid function and brain decline or dementia, screening for thyroid dysfunction (particularly subclinical thyroid dysfunction) may not be helpful when investigating patients for these conditions. Also, while the study was not designed to assess whether treating thyroid dysfunction would improve brain function, the authors felt that it was unlikely to help, particularly in those with subclinical thyroid disease.

— Philip Segal, MD

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