Background: Hyperthyroidism caused by Graves' disease is common in women, yet little is known about risk factors for the disease. We sought to determine whether lifestyle factors, including smoking, alcohol consumption, physical activity level, and body mass index, are risk factors for Graves' hyperthyroidism.
Methods: This analysis was conducted using data from the Nurses' Health Study II, among 115109 women aged 25 to 42 at entry. Incident reports of women with Graves' hyperthyroidism, confirmed to have the disorder, were included.
Results: During 1 328 270 person-years of follow-up, incident diagnoses of Graves' hyperthyroidism were confirmed in 543 women; the 12-year incidence was 4.6 per 1000 women. Cigarette smoking was a predictor of Graves' hyperthyroidism. The hazard ratio among current smokers was 1.93 (95% confidence interval [CI], 1.54-2.43), and among past smokers it was 1.27 (95% CI, 1.03-1.56), after adjusting for recent pregnancy, parity, and other variables. Among current smokers, the hazard ratio increased with the intensity of smoking and was 2.63 (95% CI, 1.71-4.04) among women who smoked 25 or more cigarettes daily. Obesity was associated with a decreased risk of Graves' hyperthyroidism. The hazard ratio for the disorder among women with a body mass index of 30 kg/m(2) or higher was 0.68 (95% CI, 0.49-0.92). Alcohol intake and physical activity level were not associated with risk of Graves' hyperthyroidism.
Conclusions: Smoking is a risk factor for Graves' hyperthyroidism in women. Obesity may be associated with a reduced risk, although weight loss as the first manifestation of hyperthyroidism cannot be excluded.