Hypothyroidism Means Less Breast Cancer Risk

Discovery Could Lead to Direction for Treatment, Research

Women with a common thyroid gland disorder appear to have a reduced chance of developing invasive breast cancer, according to a study published in the March 15 issue of the journal Cancer.

The study by M. D. Anderson researchers found that women with primary hypothyroidism (underactive thyroid) had a 61% lower risk of developing invasive breast cancer. Additionally, women newly diagnosed with breast cancer were 57% less likely to have hypothyroidism compared to a control group of healthy women.

Breast cancer patients on the study with a history of hypothyroidism tended to be older when diagnosed and had a less aggressive, indolent variety of the disease that was sensitive to estrogen treatment. The study compared the medical records of 1,136 women treated at M. D. Anderson for breast cancer to those of 1,088 healthy women who came to the institution for breast cancer screening.

Findings could lead to preventive measures

"These intriguing and very exciting findings suggest a biological role of thyroid hormone in women with breast cancer that could offer some prognostic or therapeutic value, perhaps suggesting novel preventive strategies," says Massimo Cristofanilli, M.D., associate professor in M. D. Anderson's Department of Breast Medical Oncology and the study's lead author.

The study also draws attention to the role of thyroid hormone and its potential interaction with estrogen. Estrogen controls growth of female reproductive tissues, including the breasts, and thyroid hormones control energy metabolism in tissue.

"In the past we've looked only at the role of estrogen on breast tissue to promote the onset of cancer," Cristofanilli says. "Now, we need many more studies to explore the role of other hormones."

The influence of thyroid disease on breast cancer has been debated for some time. Other smaller studies have focused on a population of women with several thyroid conditions, such as hypothyroidism, hyperthyroidism (overactive thyroid), thyroid cancer and others, and reported conflicting results on the incidence of breast cancer. Consequentially, researchers were not sure what to make of the different findings, Cristofanilli says.

Study is first regarding hypothyroidism

M. D. Anderson's study is the first clinical investigation to examine the characteristics of invasive breast cancer in patients with hypothyroidism and compare them with a control group.

"Thyroid hormone and estrogen share similar pathways in regulating growth in the target cells, including cancer cells," Cristofanilli says. "This well known phenomenon of cross-talk between the receptor of these hormones may promote or inhibit, thereby determining the "fate" of a cell towards either a regulated growth or a cancer.

"If results of a planned M. D. Anderson prospective study bear out this conclusion, then it may be possible to design a treatment that targets thyroid hormone receptors, which might help prevent breast cancer. It might also serve as a complement to tamoxifen," Cristofanilli says.

Even before this research is concluded, Cristofanilli believes that all women diagnosed with breast cancer should have their thyroid tested for hyper- or hypothyroidism, so that women with both conditions can be closely monitored.