Underactive Thyroid Lowers Risk for Breast Cancer
Women with a common thyroid problem called hypothyroidism are less likely to develop invasive breast cancer than women with normal thyroid function, according to a study reported in the medical journal Cancer.
Hypothyroidism, a condition estimated to affect about 20 percent of older women, is produced when the thyroid gland, located in the front of the neck, fails to produce enough thyroid hormone.
Insufficient levels of specific types of thyroid hormone may affect all body functions, and can cause mental and physical sluggishness.
The new study found circulating thyroid hormone may promote more aggressive forms of breast cancers, suggesting a significant role for thyroid hormone in breast cancer biology.
Breast cancer is the most common cancer among women, except for nonmelanoma skin cancers. The chance of developing invasive breast cancer at some time in a woman’s life is about 1 in 7 (13.4 percent), according to the American Cancer Society.
It is estimated that in 2005 about 211,240 new cases of invasive breast cancer will be diagnosed among women in the US. At this time there are slightly over 2 million women living in the US who have been diagnosed with and treated for breast cancer.
Carcinoma in situ (CIS) accounts for about 58,490 new cases each year. CIS is noninvasive and is the earliest form of breast cancer.
Breast cancer also occurs in men. An estimated 1,690 cases will be diagnosed in men in 2005.
Over a century ago physicians described using thyroid extract to treat breast cancer. Later, studies found that one of the circulating thyroid hormones actually sustained breast cancer cells, while animal studies found mammary gland cells responded to thyroid hormones.
Population studies have shown conflicting patterns in the relationship between thyroid hormone and breast cancer.
For example, Japanese scientists found patients with thyroiditis - an elevated thyroid hormone state - had a higher incidence of breast cancer, while other scientists found high levels of thyroid hormones were protective against breast cancer.
Studies of benign thyroid disorders have also failed to demonstrate associations between thyroid hormones and breast cancer biology.
To investigate and clarify the relationship between thyroid levels and the risk of breast cancer, Dr. Massimo Cristofanilli and colleagues at the University of Texas MD Anderson Cancer Center in Houston compared the medical records from 1,136 women with breast cancer and 1,088 healthy women attending their breast screening clinic.
The researchers found women with primary hypothyroidism had a 61 percent lower risk of developing invasive breast cancer. In addition, women with breast cancer were 57 percent less likely to have hypothyroidism compared to healthy women.
Analysis of breast tumor pathology showed that women with hypothyroidism were diagnosed with earlier stage disease and smaller tumor size.
"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," says Dr. Cristofanilli.
“The study also draws attention to the role of thyroid hormone and its potential interaction with estrogen to promote the onset of breast cancer,” Dr. Cristofanilli says.
“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,” he says.
The influence of thyroid gland disease on breast cancer has been debated for some time.
Other smaller studies have focused on a population of women with several thyroid conditions, such as hypothyroidism, hyperthyroidism (over-active thyroid), thyroid cancer, and others, and reported conflicting results on the incidence of breast cancer.
Researchers were not sure what to make of the different findings, Dr. Cristofanilli explains.
Even before this research is concluded, Dr. 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.
Always consult your physician for more information.
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