Cigarette smoking and the risk of thyroid cancer.

Thyroid cancer, whose aetiology is largely uncertain, has been negatively associated with cigarette smoking in a number of studies, possibly consistent with the greater occurrence of the disease in women than in men. This association was explored in the context of a Canadian case-control study of thyroid cancer. Newly diagnosed cases were identified primarily through provincial cancer registries in Canada and controls were identified from the general population. Data were collected through mailed questionnaires, yielding 1224 cases and 2659 controls. Reduced risk was observed for ever/never cigarette smoking (risk ratio estimate (RR) of 0.71 (95% confidence interval (CI)=0.60-0.83) for females and 0.77 (95% CI=0.58-1.02)) for males. Dose-response effects were observed with duration, quantity smoked and pack-years of exposure, although there was no decreasing protective effect with the age started smoking or years since stopped smoking. There is evidence of reduced risk for all histological subgroups. The protective effect of smoking may be due to a number of different mechanisms, including an effect on thyroid stimulating hormone and on oestrogen metabolism.