Women with primary hypothyroidism seem to have a reduced risk for invasive breast cancer compared with women who have normal thyroid glands, according to an article published online February 14th by the journal Cancer.

In a retrospective case-control study of 2,226 women, American researchers found that women with primary hypothyroidism had a 61-percent lower risk for developing invasive breast cancer. Women newly diagnosed with breast cancer were 57-percent less likely to have hypothyroidism compared with a control group of healthy women.

In addition, the breast cancer patients in the study who also had a history of hypothyroidism tended to be older when diagnosed and have a less aggressive, indolent variety of the malignancy that was sensitive to estrogen treatment.

"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," said Massimo Cristofanilli, MD, the study's lead author. "The study also draws attention to the role of thyroid hormone and its potential interaction with estrogen to promote the onset of breast cancer. We need to consider that while in the past we've looked only at the role of estrogen on breast tissue to promote the onset of cancer - thereby promoting research that brought therapeutic and preventive hormonal approaches to the patient - many more studies need to be done to explore the role of other hormones."

The influence of thyroid gland disease on breast cancer has been debated for some time. Other smaller studies have focused on women with several thyroid conditions, such as hypothyroidism, hyperthyroidism, thyroid cancer; they have reported conflicting results on the incidence of breast cancer. Consequentially, researchers were not sure what to make of the different findings, Cristofanilli said.

M. D. Anderson's study is the first clinical investigation to examine the characteristics of invasive breast cancer in patients with hypothyroidism and compare the incidence of this common condition with a carefully selected matched control group. The study compared the medical records of 1,136 patients with those of 1,088 healthy women who came for breast cancer screening.

"Thyroid hormone and estrogen both share similar pathways in regulating proliferation and growth in the target cells, including cancer cells. 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," he said.

"If results of a planned M. D. Anderson prospective study bear out this conclusion, then it may be possible to design a treatment that specifically and narrowly targets thyroid hormone receptors, which might provide enough influence on the target cells to help prevent breast cancer - perhaps even serving as a complement to tamoxifen," Cristofanilli said.

Even before this research is concluded, Cristofanilli believes that, as a first step, all women diagnosed with breast cancer should have their thyroid function tested to detect common disorders - namely hyper- or hypothyroidism - so that women with both conditions can be closely monitored.