Women who smoke are twice as likely to develop an overactive thyroid caused by Graves' disease, a study shows.

Researchers say the results suggest that cigarette smoking should be considered a major risk factor for the mysterious disorder.

Graves' disease is a common cause of an overactive thyroid (known as hyperthyroidism), but researchers say little is known about what risk factors are associated with the disease. It affects up to one in 1,000 women.

What is Hyperthyroidism?

Smoking Doubles Graves' Disease Risk

Studies in twins suggest that genetics play a major role in Graves' disease risk. But environmental and lifestyle risk factors have also been proposed, including cigarette smoking, stress, and traumatic life events. Some studies have also suggested that alcohol use may have a protective effect.

In this study, published in the current issue of the Archives of Internal Medicine, researchers looked at the effect of lifestyle factors on the risk of Graves' disease in more than 115,000 women.

Researchers analyzed information provided by the participants.

During the follow-up period, 543 of the women developed Graves' disease.

The results showed that women who currently smoked were nearly twice as likely to have Graves' disease compared with nonsmokers.

The more women smoked, the more likely they were to develop Graves' disease. Heavy smokers (more than 25 cigarettes per day) were nearly three times more likely to have the disease.

The risk of Graves' disease declined dramatically 10 to 15 years after quitting smoking. But even past smokers were slightly more likely to develop the condition.

Understanding Graves' Disease -- Symptoms

Narrowing the Search for Graves' Disease Risk Factors

None of the other lifestyle factors studied -- physical activity, obesity, or alcohol use -- was associated with an increase in Graves' disease risk among the women.

But the study suggests that obesity may have a slightly protective effect against Graves' disease. Women with a BMI (body mass index) over 30, which is considered obese, were 32 percent less likely to have the disease two years later.

Researchers say this finding should be interpreted with caution because weight loss is an early symptom of hyperthyroidism caused by Graves' disease. In fact, when researchers looked at BMI four years before diagnosis of Graves' disease, this association was no longer significant.

Ten Little-Known Reasons to Quit Smoking

By Jennifer Warner, reviewed by Michael W. Smith, MD

SOURCE: Holm, I. Archives of Internal Medicine, July 25, 2005; vol 165: pp 1606-1611.