A new study found that women with and overactive thyroid gland - hyperthyroidism - have 11 percent increased risk for breast cancer compared to women whose thyroid glands are functioning normally.

In contrast, hypothyroidism is the opposite condition where the thyroid gland produces very low levels of thyroid hormones. Women who have hypothyroidism have 6 percent reduction in breast cancer risks compared to women with normal-functioning thyroids.

A team of Danish researchers analyzed the data of almost 80,000 women with hyperthyroidism and over 61,000 women with hypothyroidism. At the start of the study, all the Danish women were cancer-free and they were followed for 36 years from 1978 until 2013. The research team monitored the women's breast cancer occurrence every five to seven years.

Lead author Dr. Jens Otto Lunde Jorgensen said the findings weren't as surprising despite the fact that the patients were tracked longer than other similar efforts. Jorgensen mentioned that the link between hyperthyroidism had been documented in other studies. He also mentioned that despite the association, hyperthyroidism doesn't necessarily cause breast cancer or vice versa and that there are many potential explanations for the link.

"Women with thyroid disease are more likely to see their doctor and to undergo examinations including mammography," said Jorgensen, an internal medicine and endocrinology clinical professor at Denmark's Aarhus University Hospital.

The lead author highlighted that thyroid hormones could be playing a part in breast cancer development and that hyperthyroidism treatments could increase the risk. The research was published in the European Journal of Endocrinology on Feb. 11.

"High levels of thyroid hormone levels can have estrogen-like effects, which may explain why hyperthyroidism is associated with higher risk of breast cancer," said Dr. Mette Søgaard, who was part of the research team.

The team highlighted that women are more inclined to have hyperthyroidism. For every 100,000 people, 51 are diagnosed with the clinical condition. Jorgensen said that despite the increasing practice of thyroid cancer tests, their study findings do not validate increased hyperthyroidism screenings.

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