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.