Underactive Thyroid Cuts Breast Cancer Risk

Hypothyroidism Also Linked to Less Aggressive Breast Cancer

Medically Reviewed by Brunilda Nazario, MD on February 11, 2005
From the WebMD Archives

Hypothyroidism Also Linked to Less Aggressive Breast Cancer

Feb. 11, 2005 -- 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.

A Thyroid-Breast Cancer Link?

The study isn't the first to explore the thyroid's impact on breast cancer. A hundred years ago, doctors tried using thyroid extracts to treat patients with breast cancer that had spread beyond the breast, say the researchers.

That approach fell out of favor, and the idea of a thyroid-breast cancer connection remains controversial. Over the years, studies have yielded conflicting results.

Cristofanilli and colleagues studied about 1,100 female breast cancer patients and 1,000 without breast cancer.

Most participants were white (77%). All were about 51 years old. Both groups had comparable percentages of women who had gone through menopause.

Hypothyroidism was identified in 272 (11%) women. All but 30 had previously been diagnosed with hypothyroidism and were taking thyroid replacement therapy.

Less Breast Cancer With Underactive Thyroid

Hypothyroidism wasn't equally spread out among the two groups. It affected nearly 15% of the women without breast cancer, compared with 7% of the breast cancer patients.

Breast cancer patients were 57% less likely to have hypothyroidism than those without breast cancer.

What's more, women with hypothyroidism had a 61% lower risk of developing invasive breast cancer.

Next, the scientists took a closer look at the 78 breast cancer patients who were white and who also had hypothyroidism.

Those women were older when their breast cancer was diagnosed. They were nearly 59 years old at the time of diagnosis, compared with 51 years for breast cancer patients without hypothyroidism.

They were also more likely to have been diagnosed with smaller, less aggressive breast cancers.

"The results of this study demonstrated that women with a diagnosis of primary, symptomatic hypothyroidism who are on thyroid supplementation are less likely to be diagnosed with invasive breast [cancer]," write the researchers.

More work should be done on the topic, they conclude.

Show Sources

SOURCES: Cristofanilli, M. Cancer, March 15, 2005; vol 103. WebMD Medical Reference from Healthwise: "Hypothyroidism: Topic Overview." WebMD Medical News: "Underactive Thyroid Lowers Breast Cancer Risk." WebMD Medical Reference from Healthwise: "Hypothyroidism: Exams and Tests." News release, John Wiley & Sons, Inc.

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