Results Summary

What was the research about?

People who are transgender have a gender identity that differs from the male or female sex assigned to them at birth. Some transgender people get therapies to change their bodies to match their gender identity. For transgender women, therapy may include taking hormones such as estrogen.

This research project earlier showed that transgender women who took estrogen had a higher risk of blood clots and strokes than men and women who weren’t transgender. In this study, the research team looked further to learn how different types of hormone therapy affect those risks. The team looked at whether taking estrogen as a pill, a shot, or a patch affected the risk. They also looked at the risks of taking estrogen along with different medicines that decrease testosterone.

What were the results?

Overall, transgender women who took estrogen had a higher risk of blood clots and strokes than men and women who weren’t transgender.

Compared with men and women who weren’t transgender, the risk of blood clots was

  • Higher in transgender women who took estrogen as pills
  • About the same in transgender women who took estrogen as shots or patches
  • Lower in transgender women who took spironolactone rather than other medicines used to lower testosterone

Also, the risk of strokes was

  • Higher in transgender women who took estrogen as pills, shots, or patches
  • About the same no matter which medicine transgender women took to lower testosterone

Who was in the study?

The research team looked at the health records of 3,325 transgender women and 62,033 men and women who weren’t transgender. All were members of Kaiser Permanente health systems in California. Of these, 55 percent were White, 12 percent were Asian, and 6 percent were Black. Also, 20 percent were Hispanic, and 64 percent were ages 18–36.

What did the research team do?

Using health records, the research team compared the risk of blood clots and strokes in transgender women taking hormone therapy versus men and women who weren’t transgender.

Transgender people, advocates, doctors, and health system staff worked with the research team during the earlier study.

What were the limits of the study?

This study looked at health records from patients in one health system in one state. Hormone therapy may vary across people and health providers. As a result, findings may differ for other patients.

Future research could continue to look at the risks of blood clots and strokes among transgender women. Studies could look at standardized hormone therapy.

How can people use the results?

Transgender women and their doctors can consider the results when starting hormone therapy. They can also look for signs of blood clots and strokes when taking hormone therapy.

Final Research Report

View this project's final research report.

More to Explore...

Related PCORI-Funded Research Project

Examining Health Outcomes for People Who Are Transgender

Peer-Review Summary

Peer review of PCORI-funded research helps make sure the report presents complete, balanced, and useful information about the research. It also assesses how the project addressed PCORI’s Methodology Standards. During peer review, experts read a draft report of the research and provide comments about the report. These experts may include a scientist focused on the research topic, a specialist in research methods, a patient or caregiver, and a healthcare professional. These reviewers cannot have conflicts of interest with the study.

The peer reviewers point out where the draft report may need revision. For example, they may suggest ways to improve descriptions of the conduct of the study or to clarify the connection between results and conclusions. Sometimes, awardees revise their draft reports twice or more to address all of the reviewers’ comments. 

Peer reviewers commented and the researchers made changes or provided responses. Those comments and responses included the following:

  • A reviewer expressed concern that this project did not seem to formally engage with stakeholders, including transgender women. The researchers explained that this project is a relatively modest extension of the Examining Health Outcomes for People Who Are Transgender research study, which was driven by stakeholder input. The researchers felt that the data analysis undertaken for this report did not require a separate stakeholder engagement component. Instead, they discussed the analysis plans for this project with the advisory group for the original research study, which includes four transgender women.
  • A reviewer noted that there are very few studies that examine the effects of hormone therapy on health in the transgender or gender non-conforming communities and said the data in this report could be presented in ways that would be clearer to such patients. The researchers agreed about the dearth of such studies and noted that an advantage of the original research study is that it allows for additional studies to address multiple research questions. While this report was written for a scientific audience, the researchers said they will begin to disseminate their findings to the broader stakeholder community using a different type of communication.

Conflict of Interest Disclosures

Project Information

Michael Goodman, MD, MPH
Emory University
Acute Thrombotic Events Following Feminizing Hormone Therapy

Key Dates

December 2022


Has Results
Award Type
Health Conditions Health Conditions These are the broad terms we use to categorize our funded research studies; specific diseases or conditions are included within the appropriate larger category. Note: not all of our funded projects focus on a single disease or condition; some touch on multiple diseases or conditions, research methods, or broader health system interventions. Such projects won’t be listed by a primary disease/condition and so won’t appear if you use this filter tool to find them. View Glossary
Populations Populations PCORI is interested in research that seeks to better understand how different clinical and health system options work for different people. These populations are frequently studied in our portfolio or identified as being of interest by our stakeholders. View Glossary
Intervention Strategy Intervention Strategies PCORI funds comparative clinical effectiveness research (CER) studies that compare two or more options or approaches to health care, or that compare different ways of delivering or receiving care. View Glossary
State State The state where the project originates, or where the primary institution or organization is located. View Glossary
Last updated: October 16, 2023