Incidence of thyroid cancer in women in relation to known or suspected risk factors for breast cancer.

There is evidence to suggest that breast and thyroid tumors occur together in the same woman more often than would be expected by chance. This study investigates the possibility that various known risk factors for breast cancer also influence the risk of thyroid cancer in women. Female residents of western Washington, in whom papillary, follicular, or mixed thyroid cancer had been diagnosed between 1974 and 1979 (N = 182), were interviewed regarding their medical and reproductive histories. For comparison, a random sample of 389 women from the same population were interviewed. Women who had a history of breast cancer were almost 3 times (95% confidence interval, 0.78-7.9) more likely to develop thyroid cancer than women with no such history. However, a history of breast cancer in a woman's mother did not increase her risk of thyroid cancer. Neither nulliparity, infertility, late age at first full-term pregnancy, early age at menarche, nor a history of abortion or miscarriage before first full-term pregnancy appeared to influence the occurrence of thyroid cancer. Increased weight was associated with an increased risk of thyroid cancer; relative to women who weighed 52 kg or less, those who weighed 60 kg or more had a 2.5-fold elevation in risk. These findings suggest that while cancers of the breast and thyroid are epidemiologically similar in a few ways, there are important differences in a number of their risk factors.