Young adults with overactive thyroids are at increased risk of stroke, a new study suggests.

But because a young person’s overall likelihood of having a stroke is small - less than 1 percent of individuals with normal thyroid function in the current study had strokes over a five-year period - the risk of stroke associated with hyperthyroidism in young people remains small.

Located at the front of the neck, the thyroid gland is responsible for regulating a number of key body processes, including metabolism. Hyperthyroidism occurs when the gland releases hormones in excessive quantities. The condition may be treated with drugs, surgery, or radiation.

Older people with hyperthyroidism are known to be prone to abnormal heart rhythms that can increase stroke risk, Dr. Herng-Ching Lin of Taipei Medical University in Taiwan and colleagues point out in the journal Stroke. Hyperthyroidism can cause excessive blood clotting and dysfunction of cells lining the blood vessels, which could make stroke more likely. At the same time, a substantial proportion of strokes that occur in young people have no known cause.

To investigate whether hyperthyroidism might help explain stroke in young people, Lin and colleagues followed 3,176 men and women with hyperthyroidism and 25,408 without the condition for five years. All of the study participants were 18 to 44 years old at the study’s outset.

During follow-up, 198 study participants suffered ischemic strokes, which are due to blocked blood flow in the brain. This translated to an overall five-year stroke risk of 0.7 percent. One percent of the hyperthyroid patients had stroke during follow-up, compared to 0.6 percent of the control group.

Once the researchers adjusted for some factors that could influence stroke risk such as age, high blood pressure, abnormal heart rhythm, and heart disease, they found people with hyperthyroidism were 44 percent more likely to have had an ischemic stroke.

The researchers emphasize, however, that they did not have access to information on smoking, alcohol consumption, dietary factors, body weight, oral contraceptive use, and family history - all of which may contribute to stroke risk.

The current findings, they conclude, hint that hyperthyroidism could be a risk factor for stroke in young people, although a young person’s absolute risk of suffering a stroke, with or without hyperthyroidism, remains small.

SOURCE: Stroke, online April 1, 2010.