Transmasculine people's blood pressure increased after starting hormone therapy, while the opposite trend was seen in transfeminine participants, according to the largest and longest study of its kind.

In the study, which published April 19 in Hypertension, researchers tracked the blood pressure of 470 gender-diverse people for up to 57 weeks after they started gender-affirming hormone therapy. They say the findings highlight a need to monitor blood pressure in people starting such treatments.

High blood pressure, or hypertension, forces the heart to work harder, boosting the risk of cardiovascular disease such as heart failure and heart attacks. Previous studies have shown that transgender people tend to have higher blood pressure and greater risk of heart disease than their cisgender counterparts. But it's not clear how gender-affirming hormone therapy, which is used to help align body characteristics with a person's gender identity, contributes to such health problems.

"We have very limited data on the effects of hormone therapy in trans individuals in regards to things like blood pressure," said senior study author Michael Irwig, an associate professor of medicine at Harvard Medical School and director of transgender medicine at Beth Israel Deaconess Medical Center in Boston. "What the study attempted to do is to look at blood pressure readings before starting therapy and see whether they changed after starting estrogen and antiandrogen for trans women or testosterone for transmasculine patients."

Transmasculine people are those who were assigned female at birth but identify as men or with masculinity, while transfeminine people were assigned male at birth but identify as women or with femininity. 

For their study, Irwig and his colleagues followed 247 transfeminine and 233 transmasculine participants. The researchers measured baseline blood pressure before people began hormone therapy and also at several different times after they started treatment.

The researchers found no noticeable difference in diastolic blood pressure, a measure of how much force blood exerts on arteries when the heart is relaxed. But within two to four months of participants starting treatment, the researchers saw changes in their systolic blood pressure, the force that blood exerts when the heart contracts. For transfeminine patients, systolic blood pressure dropped by 4 millimeters of mercury, and for transmasculine patients, it rose by 2.6 millimeters of mercury.

"The fact that it happened so quickly after starting the hormones makes me quite confident that there's a high probability that this is due to the hormones and not something else that just happened," Irwig told The Academic Times. "For example, that would be too short of a time for body weight to change a lot."

Other studies have found that transgender people not undergoing hormone treatment have higher blood pressure than cisgender people and greater rates of cardiovascular disease. According to Irwig, this disparity could be due to differences in lifestyle, such as obesity, exercise and smoking, or psychological factors such as depression and stress.

The new findings suggest that some gender-diverse people undergoing hormone therapy, particularly transmasculine people taking testosterone, could be at greater risk of conditions such as stroke or heart attack. 

But not all participants responded the same way to hormone therapy.

"For transfeminine individuals, although the majority had a decrease in their systolic blood pressure, about a quarter actually had an increase in their systolic blood pressure," Irwig said. "And likewise for transmasculine patients; although the average went up, about a quarter actually saw their systolic blood pressure go down."

Overall, the study points to the importance of monitoring blood pressure in people who choose to have gender-affirming hormone therapy, according to Irwig.

"For physicians treating patients, it's important to measure blood pressure before starting the hormones, and if people do have confirmed high blood pressure, to manage that appropriately with diet, lifestyle, weight loss or medications," he said. "After starting hormone therapy, it's important to continue monitoring blood pressure to make sure that patients don't have such a rise in their blood pressure that could put them into the hypertension range."

Although the results suggest that hormone therapy was responsible for changes in blood pressure seen in the participants, other explanations cannot be ruled out because the researchers did not include a comparison group of gender-diverse people who did not undergo hormone therapy, Irwig explained. 

Another limitation of the study is that most participants were on the same type of oral estrogen or intramuscular testosterone, so it's unclear whether other formulations are similarly linked with blood pressure changes.

"There's different ways to give testosterone: there's injections, there's topical gels, there's patches, there's pills, there's implants. And the same thing with estrogen: Estrogen comes in pills, people can dissolve it on their tongue, put a patch on, take injections," Irwig said. "It would be interesting to look at different formulations of hormone therapies to see if they have the same effects."

The study, "Blood pressure effects of gender-affirming hormone therapy in transgender and gender-diverse adults," published April 19 in Hypertension, was authored by Katherine Banks and Mabel Kyinn, The George Washington School of Medicine & Health Sciences; Shalem Y. Leemaqz, Flinders University College of Medicine and Public Health; Eleanor Sarkodie and Deborah Goldstein, Whitman-Walker Institute; and Michael S. Irwig, The George Washington School of Medicine & Health Sciences and Beth Israel Deaconess Medical Center & Harvard Medical School.