A new study says some IUDs can affect breast tissue — challenging a myth that the birth control devices only work locally

• Hormonal IUDs may have effects far outside the uterus, even affecting breast tissue, study suggests.
• The findings challenge common assumptions that the devices only work locally, in the uterus.

Hormonal IUDs like Mirena and Jaydress can have full-body effects noticeable in breast imaging scans even though they work by releasing hormones directly into the uterus, a not-yet-published study out of Germany suggests.

The finding counters the popular understanding that IUDs work purely locally, lead author Dr. Luisa Huck of RWTH Aachen University's department of diagnostic and interventional radiology, said in a Q&A document for reporters. "Our study results suggest that this is not true."

In fact, the researchers found the IUDs' full-body hormonal effects were similar to those of hormone replacement therapy, a press release says. They say their findings show it's "plausible that IUDs can have side effects similar to that of other hormonal treatments," as women have long reported.

Dr. Amy Roskin, an OB-GYN and Chief Medical Officer of The Pill Club who was not involved in the study, told Insider she's not surprised by the findings, which were presented last month at the Radiological Society of North America's annual meeting.

"We already know that although the primary effect of the levonorgestrel IUD hormone is on the uterus, some of the hormone is absorbed and will impact other tissue and areas," she said, adding that the level absorbed systemically should still be lower than with other birth control methods like the pill.

While manufacturers like Bayer don't claim IUDs completely spare the rest of the body from hormones or their side effects, they say their devices have "mainly local progestogenic effects."

The study authors say their work is important to validate women's reports of full-body side effects from IUDs, and to help patients and clinicians make informed decisions about contraception.

The researchers looked at breast scans before and after IUD insertion and removal

Co-author Dr. Christiane Kuhl, chief of the Department of Radiology at RWTH Aachen University, wanted to look at IUDs' effects on breast tissue after noticing that women who used them tended to have a higher background parenchymal enhancement (BPE) on contrast-enhanced breast MRI.

BPE is a sensitive marker of hormone levels.

To conduct the study, she and colleagues identified 48 premenopauseal women in a hospital database who had no history of breast cancer or hormone or antihormone intake, and who had undergone a contrast-enhanced breast MRI at least twice.

Half of the women underwent the first breast MRI exam before they got their IUDs and the second once the IUDs were inserted. The other half had their first MRI with an IUD and the second after it was taken out.

This method allowed them to see how IUDs affected each women's BPE levels and avoid any age-related hormonal changes that could have muddied the results.

The researchers found that BPE levels went up significantly in 23 of 48 patients.

Not only does that suggest IUDs' effects can be detected far from the uterus, but it also means IUD use should be considered when interpreting the results of breast MRI, the study authors say.

"IUDs appear to be a very safe means of contraception, and are generally well tolerated," Huck wrote to reporters. "However, in case you experience so-far unexplainable side effects, talk to your doctor, and consider using other types of contraception."

For example, Roskin said the copper IUD is hormone-free.