Smoking and thyroid disorders--a meta-analysis.

BACKGROUND Smoking has been associated with Graves' disease, but it remains unclear if the association is present in other thyroid disorders. OUTCOME VARIABLES Graves' disease, Graves' ophthalmopathy, toxic nodular goitre, non-toxic goitre, post-partum thyroid disease, Hashimoto's thyroiditis, or hypothyroidism. MATERIAL AND METHODS A search of MEDLINE identified 25 studies on the association between smoking and thyroid diseases. RESULTS In Graves' disease eight studies were available showing an odds ratio (OR) of 3.30 (95% confidence interval (CI): 2.09-5.22) in current smokers compared with never smokers. In ex-smokers there was no significant excess risk of Graves' disease (OR=1.41, 95% CI: 0.77-2.58). The OR associated with ever smoking in Graves' ophthalmopathy (4.40, 95% CI: 2.88-6.73, six studies) was significantly higher than in Graves' disease (1.90, 95% CI: 1.42-2.55, two-sided P-value <0.01). Ever smoking was not associated with toxic nodular goitre (OR=1.27, 95% CI: 0.69-2.33, three studies), while there was an increased risk of non-toxic goitre in smokers if men were excluded (OR=1.29, 95% CI: 1.01-1.65, eight studies). The risk associated with smoking was significantly lower in men than in women for both Graves' disease and non-toxic goitre. Hashimoto's thyroiditis and post-partum thyroid dysfunction were also associated with smoking while the association with hypothyroidism did not reach statistical significance. CONCLUSIONS Cessation of smoking seems associated with a lower risk of Graves' disease than current smoking. Smoking increases the risk of Graves' ophthalmopathy beyond the risk associated with Graves' disease alone. Smoking cessation may lead to a decrease in morbidity from Graves' disease, especially in women.