Millions of older adults try to manage an underactive thyroid gland with daily medication, but a new study suggests they may still be vulnerable to developing dementia as they age.

Researchers found that among over 15,000 older Taiwanese adults, those who'd been treated for hypothyroidism were substantially more likely to be diagnosed with dementia. Hypothyroidism occurs when the body is not producing enough thyroid hormones, which control metabolism.

That underproduction can cause an array of symptoms, including fatigue, intolerance to cold, constipation, body aches, depression and weight gain. Studies show that waning thyroid hormones may also dull a person's thinking skills — in a way that's reversible with thyroid medication.

But whether hypothyroidism can contribute to irreversible dementia has been unclear. And the new study, published recently in the journal Neurology, does not answer that question definitively.

"We found that for patients with hypothyroidism that required medication, the risk of dementia was much higher," said researcher Dr. Chien-Hsiang Weng, of Brown University's Alpert Medical School, in Providence, R.I.

"But it's hard to know why," he stressed. "We can't really say that hypothyroidism causes dementia."

In addition, the vast majority of patients with dementia in the study had no history of diagnosed hypothyroidism: Just under 1% did.

Still, Weng said, it's useful to know that older people with hypothyroidism — for whatever reason — may have a heightened likelihood of developing dementia. That way, patients and doctors can be on the lookout for potential signs of declines in memory and thinking.

"We wanted to let clinicians know that hypothyroidism might be one of the risk factors," Weng said.

The findings are based on medical records from over 15,000 older adults, half of whom had been diagnosed with dementia. Each of those patients was compared with a dementia-free patient of the same sex and age.

Among dementia patients, the study found, 0.9% had a history of hypothyroidism, versus 0.4% of those without dementia.

After the researchers weighed other factors — including any history of heart disease, diabetes, depression or alcohol dependence — they found that hypothyroidism, itself, was linked to a higher dementia risk.

Among patients aged 65 and older, those with a history of hypothyroidism were 81% more likely to be diagnosed with dementia. The difference was even greater when the researchers looked at patients who'd been given hormone-replacement medication for the condition: Their dementia risk was threefold higher, versus people without hypothyroidism.

"We don't think it has anything to do with the medication itself," Weng noted. "We think medication use is a marker of more severe hypothyroidism."

The study uncovered no connection between dementia risk and hyperthyroidism, or overactivity in the thyroid gland.

A dementia expert who was not involved in the study emphasized that it cannot pin the blame on hypothyroidism.

"More work is needed to understand what this may mean, if anything, in terms of biological mechanisms and impact on [dementia] risk," said Heather Snyder, vice president of medical and scientific relations at the Alzheimer's Association.

Snyder noted that other research has been digging into the potential role of thyroid hormones in mental acuity and dementia risk.

"Some recent studies have examined whether a thyroid hormone called T3 may be linked to biological processes in Alzheimer's," she said.

The hormone apparently declines as people age, and lower T3 levels might promote the generation of beta-amyloid — a protein that builds up in the brains of people with Alzheimer's.

"To date, however, scientists have not clarified the exact relationship between abnormal thyroid hormone levels and dementia risk," Snyder said.

For now, she and Weng both said people should be aware that thyroid health is important to overall health, and talk to their doctor if they have potential symptoms of underactive thyroid.