The thyroid gland is central to metabolic control. Women are much more likely than men to have an overactive thyroid. About 51 out of every 100,000 people are diagnosed with an overactive thyroid annually.
Researchers found that women with hyperthyroidism appeared to face an 11% increase in their risk for breast cancer compared to women with a normal functioning thyroid gland. Hyperthyroidism is a condition where abnormally high levels of thyroid hormones are produced
Conversely, women with hypothyroidism had a 6% decrease in risk for breast cancer compared to women with a normal functioning thyroid gland. Hypothyroidism is a condition were abnormally low level of thyroid hormones are produced.
At least one researcher not involved in the study said that she is reluctant to read too much into the study findings.
The findings stem from a 36-year review that identified almost 80,000 Danish women with an overactive thyroid and more than 61,000 women with an underactive thyroid. All the women were cancer-free when the study started in 1978.
Breast cancer incidence was then tracked for every five to seven years.
Although the study followed patients far longer than previous efforts, researchers were not surprised by the new findings. 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.
However, while an overactive thyroid was found to be associated with breast cancer risk, one does not necessarily cause the other. There are many possible explanations for this association.
Although thyroid cancer screening is an increasingly common practice, this new finding does not justify increased screening for overactive thyroid disease.
It is possible that these findings are not generalizable across other populations; the study was conducted on a very homogenous group of women. There would never be a study group like this in the U.S. where there is a lot of ethnic variation.
