Cigarette smoking and the risk of thyroid cancer

Thyroid cancer, whose aetiology is largely uncertain, has been negatively a ssociated with cigarette smoking in a number of studies, possibly consisten t with the greater occurrence of the disease in women than in men. This ass ociation was explored in the context of a Canadian case-control study of th yroid cancer. Newly diagnosed cases were identified primarily through provi ncial cancer registries in Canada and controls were identified from the gen eral population. Data were collected through mailed questionnaires, yieldin g 1224 cases and 2659 controls. Reduced risk was observed for ever/never ci garette 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. Dos e-response effects were observed with duration, quantity smoked and pack-ye ars of exposure, although there was no decreasing protective effect with th e age started smoking or years since stopped smoking. There is evidence of reduced risk for all histological subgroups. The protective effect of smoki ng may be due to a number of different mechanisms, including an effect on t hyroid stimulating hormone and on oestrogen metabolism. (C) 2000 Elsevier S cience Ltd. All rights reserved.