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    Overactive Thyroid Linked to Breast Cancer Risk

    But researchers added that their study did not prove cause-and-effect

    WebMD News from HealthDay

    By Alan Mozes

    HealthDay Reporter

    THURSDAY, Feb. 11, 2016 (HealthDay News) -- Women who have an overactive thyroid gland might be at greater risk for breast cancer, a new study suggests.

    A team of Danish researchers found that women with the condition -- called hyperthyroidism -- appeared to face an 11 percent increase in their risk for breast cancer, compared to women with a normal-functioning thyroid gland.

    On the other hand, women with the opposite problem -- a condition called hypothyroidism, where abnormally low levels of thyroid hormones are produced -- saw their risk for breast cancer dip 6 percent below that of women with normal thyroids.

    But at least one breast cancer expert not involved with the study said she was reluctant to read too much into the study findings.

    The findings stem from a 36-year review that identified nearly 80,000 Danish women with an overactive thyroid and more than 61,000 women with an underactive thyroid. All of the women were cancer-free when they entered the study, which ran from 1978 to 2013.

    Breast cancer incidence was then tracked for roughly every five to seven years.

    Study lead author Dr. Jens Otto Lunde Jorgensen said that while the review followed patients far longer than previous efforts, the new findings aren't all that surprising. In fact, he noted that "a similar association between overactive thyroid disease and [the] risk of breast cancer in women has been reported in three out of four previous studies."

    At the same time, he stressed that while an overactive thyroid was found to be associated with breast cancer risk, one does not necessarily cause the other, adding that "the possible explanations for this 'association' are many."

    "Women with thyroid disease are more likely to see their doctor and to undergo examinations including mammography," Jorgensen said. "Women with overactive thyroid disease also are predisposed to other diseases such as breast cancer, but it is not the overactive thyroid by itself. Thyroid hormones may contribute to development of breast cancer. [And] treatment of overactive thyroid disease increases the risk of breast cancer. These are all possibilities, but the bottom line is that we do not know for sure."

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