Millions of older adults try to manage an underactive thyroid gland with daily medication,

A new study suggests they may still be vulnerable to developing dementia as they age.

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.