Women with thyroid problem have lower risk of breast cancer

According to a new study, women suffering from a common thyroid problem called hypothyroidism, a condition in which the thyroid gland fails to produce enough thyroid hormone, are less likely to develop breast cancer than women with normal thyroid function. The study, published in the March 15, 2005 issue of CANCER (http://www.interscience.wiley.com/cancer-newsroom), a peer-reviewed journal of the American Cancer Society, finds that a history of hypothyroidism is associated with more aggressive forms of breast cancers, suggesting that thyroid hormone may promote breast cancer biology.

Over a century ago physicians described using thyroid extract to treat breast cancer. Later, studies found that one of the circulating thyroid hormones actually sustained breast cancer cells, while animal studies found mammary gland cells responded to thyroid hormones. Population studies have shown conflicting patterns in the relationship between thyroid hormone and breast cancer. For example, Japanese scientists found patients with thyroiditis – an elevated thyroid hormone state – had a higher incidence of breast cancer, while other scientists found high levels of thyroid hormones were protective against breast cancer. Studies of benign thyroid disorders have also failed to demonstrate associations between thyroid hormones and breast cancer biology.

To investigate and clarify the relationship between a history of hypothyroidism and the risk of breast cancer, Massimo Cristofanilli, M.D. and colleagues at the University of Texas M. D. Anderson Cancer Center in Houston compared the medical records from 1136 women with breast cancer and 1088 healthy women attending their breast screening clinic.

The researchers found women with primary hypothyroidism had a 61 percent lower risk of developing invasive breast cancer. In addition, women with breast cancer were 57 percent less likely to have hypothyroidism compared to healthy women. Analysis of breast tumor pathology demonstrated that women with hypothyroidism were diagnosed with earlier stage disease and smaller tumor size.

The authors say, "The observed association between hypothyroidism and breast cancer may be due to the biologic effect of (the thyroid hormone) T3 at the cellular level," adding that further studies of thyroid regulating hormones and intracellular iodide concentrations may "indicate areas of intervention for targeted preventive and therapeutic purposes."