A possible association between thyroid problems and breast cancer has fueled debate among doctors for more than a century—one doctor published an article about the topic in 1896. There seems to be some association between hypothyroidism, a condition in which the body does not produce enough thyroid hormones, and breast cancer, but no one agrees on whether it makes women more or less likely to get breast cancer. Scientists from the University of Texas looked at the problem to see if they could settle the debate.

What the researchers wanted to know: How does hypothyroidism affect the development of breast cancer?

What they did: The researchers reviewed the medical histories of 1,136 women who were diagnosed with breast cancer at the University of Texas's cancer center between 1997 and 1998, and 1,088 women who did not have breast cancer. They looked at these women's medical records to see if they had been diagnosed with a thyroid disorder before they were screened for breast cancer. About 10 percent of the women had been diagnosed with a thyroid disorder, and all of them were on thyroid supplements to treat that condition.

What they found: Women who did not have breast cancer were more likely to have hypothyroidism—15 percent of the healthy women had hypothyroidism versus 7 percent of the women with breast cancer. In addition, when the authors looked at the severity of breast cancer in the women, those with hypothyroidism had, in general, smaller tumors and were more likely to be at an early stage in the disease when diagnosed. The authors say this may mean that hypothyroidism has a protective effect against breast cancer

What it means to you: An underactive thyroid gland causes its own problems—weight gain and metabolic syndrome for example—but may be protective against breast cancer or make the cancer less aggressive. The researchers say that this finding could have implications for breast cancer treatment, and they urge further research on how thyroid hormones might be used to treat breast cancer in the future.

Caveats: The researchers in this study used medical records, as opposed to screening the women themselves, so there's a chance that they missed some women who had a thyroid disorder that had not been diagnosed.

Find out more: The American Thyroid Association has a booklet about hypothyroidism online in PDF form.

Read the article: Cristofanilli, M. et al. "Thyroid Hormone and Breast Carcinoma." Cancer. March 15, 2005, Vol. 103, No. 6.