Women with an underactive thyroid are less likely to develop breast cancer. For those that do get breast cancer, their chances are better that it will be less aggressive, a new study shows.

Detection of breast cancer at an earlier, less aggressive stage improves survival in women with breast cancer.

In a study of more than 2,200 women, roughly half of whom had breast cancer, an underactive thyroid was shown to protect against developing breast cancer.

The study also showed that women who developed breast cancer and who had an underactive thyroid had a less aggressive disease compared with women with a normally functioning thyroid.

The report appears in the March 15 issue of the journal Cancer. It’s the work of Massimo Cristofanilli, MD, and colleagues from the University of Texas M.D. Anderson Cancer Center.

What Is Hypothyroidism?

Thyroid hormone regulates the body’s metabolism. The butterfly-shaped thyroid gland is found near the lower front of the neck. In hypothyroidism, the thyroid gland doesn’t produce enough thyroid hormone.

Hypothyroidism affects about 2% of women and 3%-4% of the general population.

Thyroid problems are more common among women than men. Among older women, as many as one in five may have hypothyroidism, the most common thyroid problem.

Many people don’t realize they have the problem. Symptoms include feeling tired, sluggish, or weak; dry skin; coarse and thinning hair; cold skin and the inability to tolerate cold temperatures; brittle nails or a yellowish tint to skin; constipation and heavy or irregular menstrual periods that may last longer than five to seven days.

Blood tests are needed to confirm the diagnosis, and some experts suggest that everyone (especially women) get tested at age 35.