Hormone therapy may be associated with an increased risk of blood clots and stroke among transgender women, according to a new study.

The study titled "Cross-sex Hormones and Acute Cardiovascular Events in Transgender Persons: A Cohort Study" was published in the Annals of Internal Medicine on July 9.

"This is the largest study of the health of transgender individuals on hormone therapy ever done," said study co-author Dr. Darios Getahun, a research scientist at Kaiser Permanente. "Doctors and patients need to be aware of the possibility for increased health risks for transgender women."

The researchers used the Kaiser Health System to examine data on nearly 5,000 transgender patients (2,842 transgender women and 2,118 transgender men). Data on more than 97,000 cisgender patients were also used for comparison and risk assessment.

Trans women who started hormone therapy (with estrogen) were found to have higher rates of a blood clot known as venous thromboembolism, with their risk almost twice as high as cisgender men or women. However, trans men who started hormone therapy (with testosterone) had no such heightened risk.

The risk of stroke (a potential consequence of blood clots) among trans women was 9.9 times higher compared to men in the control group and 4.1 times higher compared to women in the control group. These were the figures emerging six years after the start of the therapy.

While their risk of heart attack was 2.4 times higher compared to cisgender women, they were not found to have an increased risk of heart attack when compared to cisgender men.

In terms of limitations, the study did not explore if there were any variations in health risks based on different types and doses of hormones. 

"For now, this study is the best measure we have of the risk, and it’s much better than what we’ve had before," Getahun added. He also stated that all the data was collected from medical records at Kaiser, which meant that any external treatments and medications would not be accounted for.

While the precaution is useful to note, experts emphasize that the risk is not alarming enough to stop providing therapy. Further research is needed to outline which combinations or doses have the lowest risk of adverse health effects.

"This is a great study and I’m not surprised by its conclusions," said Dr. Richard Greene, an associate professor of medicine at the New York University School of Medicine. But having treated numerous transgender patients, he added: "But my concern is that doctors who are inclined to deny hormone therapy will use this study as a reason to do so."

For many transgender patients, hormone therapy is essential as they seek to align their sex with their gender identity. Research has shown that it is highly effective at improving the mental health of trans men and women.