Underactive Thyroid Lowers Breast Cancer

Hypothyroidism Also Linked to Less Aggressive Disease

From the WebMD Archives

April 10, 2003 -- Women with an underactive thyroid appear to be somewhat protected against breast cancer, according to intriguing new studies. The findings eventually could lead to better breast cancer prevention and treatment strategies.

When the researchers compared women with and without breast cancer, they found that women with breast cancer were far less likely to also have had an underactive thyroid -- a condition called hypothyroidism -- in the past. And among the women with breast cancer, those with an underactive thyroid had much less aggressive breast cancer.

"Women with a history of hypothyroidism had many more stage I and II breast cancers and very few stage III diseases," lead researcher Massimo Cristofanilli, MD, tells WebMD. "Overall, their disease was less aggressive." He is with The University of Texas M.D. Anderson Cancer Center.

Thyroid hormone regulates the body's use of energy. An underactive thyroid is the most common thyroid problem and affects as many as one in five older women. But many of those women are unaware they have an underactive thyroid. That's why some experts recommend that everyone, especially women -- since thyroid problems are much more common in women -- get tested for a thyroid problem at age 35.

Researchers have long believed that a thyroid problem influences breast cancer, but findings from earlier studies have been inconclusive. In the latest research, M.D. Anderson investigators compared more than 1,100 women with newly diagnosed breast cancers with some 1,100 women without breast cancer. They also looked at who did -- and didn't -- have an underactive thyroid. Their findings are published in the Proceedings for the 2003 Annual Meeting of the American Association for Cancer Research.

Regardless of ethnic group, women without breast cancer were roughly twice as likely to have an underactive thyroid. Women with breast cancer were 57% less likely to have a previous diagnosis of underactive thyroid than women without cancer.

The researchers then looked more closely at a group of 80 women with breast cancer who also had an underactive thyroid. These women were much more likely than other cancer patients to have signs of less aggressive breast cancer. Women with breast cancer and an underactive thyroid were more likely to have early stage breast cancer, tended to be older, and were more likely to have estrogen-receptor positive breast cancers -- all signs of less aggressive, and more easily treated, breast cancer.

Both thyroid hormones and estrogen affect the growth of a cell, including cancer cells, he says. So a problem with one or the other of these two hormones may potentially affect the ability of the other hormone to function properly, he explains.

Cristofanilli says it is not clear whether women with overactive thyroids are at increased risk for breast cancer. He adds there are several theories as to why thyroid problems and breast cancer would be linked.

And what about treatment for an underactive thyroid? How might taking thyroid hormone affect the risk of breast cancer?

"We don't have any information to say that it is dangerous to take thyroid hormone at this point, and, actually, it doesn't seem to be the case," says Cristofanilli.

He says the link between an underactive thyroid and breast cancer likely has to do with the cause of an underactive thyroid -- although this isn't always known. But as an example, he explains that an underactive thyroid may due to the immune system mistakenly attacking and killing the thyroid gland. Thus, replacing the thyroid hormone won't affect this underlying problem, he says.

Researchers are now developing another trial looking at the association between an underactive thyroid and breast cancer -- one that should offer more conclusive findings. If results of this study bear out this conclusion, Cristofanilli says, then it may be possible to design a treatment that specifically targets thyroid hormone receptors to help prevent breast cancer.

Cancer specialist Karen Antman, MD, says if the breast cancer and underactive thyroid findings are confirmed, this should open interesting new avenues of research. Antman is president-elect of the American Association for Cancer Research.

"I think this is a very intriguing piece of research that should generate a lot of interest," she tells WebMD.

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SOURCES: Proceedings of the 2003 Annual Meeting of the American Association for Cancer Research. Massimo Cristofanilli, MD, assistant professor in Department of Breast Medical Oncology, University of Texas M.D. Anderson Cancer Center. Karen Antman, MD, Columbia University, president elect, American Association for Cancer Research and director of the cancer center at Columbia University.
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