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Women's Health

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Underactive Thyroid Lowers Breast Cancer

Hypothyroidism Also Linked to Less Aggressive Disease


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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