Although hyperthyroidism was associated with an increased risk of breast cancer, hypothyroidism – an underactive thyroid – was found to decrease the risk of breast cancer by six percent.
The findings come from a 36-year review that identified nearly 80,000 Danish women with hyperthyroidism and over 61,000 women with hypothyroidism. All the women were cancer free at the beginning of the study.
Lead author Dr. Jens Otto Lunde Jorgensen said, “Women with thyroid disease are more likely to see their doctor and to undergo examinations, including mammography. 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.”
Although the findings reveal an association between the two conditions, they do not reveal cause and effect, and Dr. Jorgensen does not believe that doctors need to increase the screening for thyroid disorders as a means to lower breast cancer risk.
The findings were published in the European Journal of Endocrinology.
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