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Breast cancer is not increased in women with most thyroid disorders

(March 2003)

The background of the study. Some studies have suggested that thyroid disorders increase a woman’s risk of breast cancer. This study assessed the relationship between thyroid disorders and breast cancer in women with breast cancer and control women.

How the study was done. The study subjects were 4572 women with breast cancer who were enrolled in a population-based case–control study carried out at five locations in the United States and 4677 control women of similar age and ethnicity living in the same counties. The women were asked if they had ever had a thyroid disorder or if they had received thyroid hormone or antithyroid treatment; if so, they were questioned in detail about their responses.

The results of the study. The women with breast cancer and control women were similar in age, race, education, weight, and reproductive history. Among the women with breast cancer, 18 percent had a history of a thyroid disorder, as compared with 17 percent of the women in the control group. The frequency of a history of hyperthyroidism, hypothyroidism, goiter, and nodules was similar in the two groups, but more women with breast cancer had a history of thyroid cancer than did control women (0.5 vs. 0.2 percent); the increased risk of breast cancer in women with a history of thyroid cancer was limited to women who had borne children. There was no relationship between thyroid hormone or antithyroid treatment, including radioiodine treatment, and breast cancer.

The conclusions of the study. Thyroid disorders are not associated with an increase in risk for breast cancer, with the exception of a very small increase in risk in women with thyroid cancer who had borne children.

The original article. Simon MS, Tang MT, Bernstein L, Norman SA, Weiss L, Burkman RT, Daling JR, Deapen D, Folger SG, Malone K, Marchbanks PA, McDonald JA, Strom BL, Wilson HG, Spirtas R. Do thyroid disorders increase the risk of breast cancer? Cancer Epidemiol Biomarkers Prev 2002;11:1574-8.

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