Press Release

Study finds risk factor for blood clots occurs in more than 10 percent of transgender men using testosterone

Washington, DC February 18, 2021
A potentially dangerous side effect of testosterone therapy for transgender men is an increase in red blood cells that can raise the risk of blood clots, heart attack or stroke, according to a new study published in the Endocrine Society’s Journal of Clinical Endocrinology & Metabolism.

Gender diverse people make up an estimated 0.6% of the U.S. population and are defined as having gender identity that is not aligned with their sex recorded at birth. Transgender men often undergo testosterone therapy as part of their gender-affirming treatment. Erythrocytosis, a condition where your body makes too many red blood cells, is a common side effect of testosterone therapy that can increase the risk of blood clots, heart attack or stroke.

“Erythrocytosis is common in transgender men treated with testosterone, especially in those who smoke, have high body mass index (BMI) and use testosterone injections,” said lead study author Milou Cecilia Madsen, M.D., of the VU University Medical Center Amsterdam in the Netherlands. “A reasonable first step in the care of transgender men with high red blood cells while on testosterone therapy is to advise them to quit smoking, switch injectable testosterone to gel, and if BMI is high, to lose weight.”

The researchers analyzed the medical and laboratory records of 1,073 transgender men using testosterone from the Amsterdam Cohort of Gender study (ACOG). Eleven percent of transgender men using testosterone showed erythrocytosis during 20 years of follow-up. However, less than one percent of these men had very high levels of red blood cells with a high risk of complications.

Other authors of the study include: Dennis van Dijk, Chantal Maria Wiepjes, Elfi Barbara Conemans, Abel Thijs and Martin den Heijer of the VU University Medical Center Amsterdam.

The manuscript received no external funding.

The manuscript, “Erythrocytosis in a Large Cohort of Trans Men Using Testosterone: A Long-term Follow-up Study on Prevalence, Determinants and Exposure Years," was published online, ahead of print.

The authors will be presenting related research at ENDO 2021, the Endocrine Society’s annual meeting.

About Endocrine Society

Endocrinologists are at the core of solving the most pressing health problems of our time, from diabetes and obesity to infertility, bone health, and hormone-related cancers. The Endocrine Society is the world’s oldest and largest organization of scientists devoted to hormone research and physicians who care for people with hormone-related conditions.

The Society has more than 18,000 members, including scientists, physicians, educators, nurses and students in 122 countries. To learn more about the Society and the field of endocrinology, visit our site at www.endocrine.org. Follow us on Twitter at @TheEndoSociety and @EndoMedia.

Media Contacts

Colleen Williams Manager, Public Relations Phone: (202)-971-3611 [email protected]

Jenni Glenn Gingery Director, Communications and Media Relations Phone: (202)-971-3655 [email protected]

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