Older people with hypothyroidism may be at increased risk of developing dementia, according to a study from Taiwan, especially if they need thyroid hormone replacement therapy. 

Researchers matched 7,843 people with newly diagnosed dementia with the same number of controls. Overall, 102 people had hypothyroidism and 133 had hyperthyroidism. Among people with dementia, 0.9 per cent had hypothyroidism compared with 0.4 per cent of controls.

Researchers found no link between hyperthyroidism and dementia. Hypothyroidism was not associated with an increased risk of dementia among people aged between 50 and 65 years but, among patients aged 65 years or older, a history of hypothyroidism was associated with an 81 per cent increased risk of being diagnosed with dementia, after adjusting for other risk factors such as sex, age, hypertension and diabetes.

The observational study does not prove that hypothyroidism causes dementia, however.

The association was even more marked – a threefold increase – among patients aged 65 years or older with a history of hypothyroidism who received thyroid hormone replacement.

“One explanation for this could be that these people are more likely to experience greater symptoms from hypothyroidism where treatment was needed,” says study author Dr Chien-Hsiang Weng, Brown University in Providence, Rhode Island.

The researchers could not, however, adjust for hypothyroidism severity. “While more studies are needed to confirm these findings, people should be aware of thyroid problems as a possible risk factor for dementia.” 

(Neurology DOI: 10.1212/WNL.0000000000200740)