Hypothyroid postmenopausal women appear to be at significantly higher risk of developing breast cancer, Argentinian researchers reported in a poster presentation at the annual meeting of the North American Menopause Society.
In the prospective study, Dr. Maria Franchina and her colleagues of the Preventive Medical Center in Buenos Aires found a high incidence of breast cancer among postmenopausal women who were newly diagnosed with autoimmune hypothyroidism or whose hypothyroidism was poorly controlled because they had stopped taking their thyroid medication.
Among a total of 180 hypothyroid women attending their clinic between January 2006 and June 2007, 33 (18%) had breast cancer, compared with 15 (5%) of 300 euthyroid women, the investigators reported.
The median age of the women was 52.9 years, and all were within 1 year of their last menstrual period.
The researchers also compared the prevalence of endometrial, thyroid, and ovarian cancers between the two groups, but did not find any significant differences in those measures.
More hypothyroid than euthyroid women had thyroid cancer, 8% vs. 1%, but this difference was not statistically significant, Dr. Franchina said. (See accompanying graphic.)
“Growing breasts require several hormones, such as prolactin, estrogen, progesterone, adrenal steroids, insulin, growth hormone, and thyroid hormones, so there could well be an association between low thyroid hormone and breast cancer,” she said in a telephone interview.
But she added that such an association is controversial.
“There have been many discrepant results reported in the literature, with some researchers reporting that thyroid diseases are common in women with breast cancer, and others reporting just the opposite.”
In truth, whether thyroid autoimmune diseases actually increase a woman's risk for developing cancer is still not known, Dr. Franchina concluded.
The study generated a lot of interest among North American Menopause Society delegates, Dr. Franchina said.
She and her associates are planning a correlative study to further investigate this finding.
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