The most important lens for this article is that “hormones affect many body systems” is well established, but the strength of evidence varies a lot from one claim to the next. Some effects discussed here are already familiar in clinical practice, such as changes in bone health, muscle mass, body composition, hot flashes and some aspects of sexual function. Others, especially around the gut microbiome, sensory changes, brain imaging and sleep timing, are earlier-stage findings that are interesting without yet proving long-term health outcomes.
That distinction matters because readers can easily confuse measurable biological change with clear clinical benefit or harm. A shift in immune cells, gut bacteria or brain receptor density may be real, but it does not automatically tell us how a person will feel, function or fare over time. Small studies, short follow-up and population bias also limit how widely some findings can be applied.
For readers, the practical takeaway is less about any single “surprising” effect and more about how broad hormone therapy’s reach can be. These treatments do not act on one symptom alone; they can influence sleep, skin, cardiovascular markers, sexual health and recovery from exercise at the same time. That makes monitoring important, especially when balancing desired benefits against trade-offs such as acne, hair loss, vaginal dryness, bruising, changes in libido or possible effects on sleep apnea and infection risk.
Because responses differ by age, dose, route of administration, genetics and underlying health, the article is best read as a map of possibilities rather than a prediction for every individual. The real value of this research is not that hormones affect everything equally, but that better care depends on tracking which changes are meaningful, for whom and under what circumstances.
The politics of hormone therapy depend on who’s taking it. But whether hormones are taken to treat menopause or gender dysphoria, we’re talking about the same drugs. They have powerful effects on our bodies. And in recent years, research has expanded our understanding of what they do.
They can change the way our brains work, change how we sense the world, and may even lower the risk of certain cancers.
They can cause profound changes that blur the line between “biological sex” and gender identity.
The science and the future of hormones are evolving quickly, and that has far-reaching implications. Some of the links between hormones and changes in the body are well-researched; others are less so, with studies into people of different races, backgrounds and socioeconomic classes lacking. But understanding how hormones affect the body can improve healthcare for everyone. For example, some experts believe that more research into hormones and gut bacteria can advance treatment for autoimmune diseases.
We all have hormones; everyone naturally has both estrogen and testosterone. Hormone therapy changes that mix and impacts your body in similar ways, whether you’re trans or cisgender, nonbinary or not.
As more cis people start hormones and transgender people navigate limited access under the Trump administration, The 19th takes a look at how hormones can change your body.
Your immune system
A 2024 study found that, among trans men, testosterone alters immune cells. After a year of hormone therapy, study participants’ immune systems resembled that of cis men — making them slightly more susceptible to viral infections.
And for menopausal women, estrogen taken through hormone therapy may boost immune health, according to a study released last fall from the Queen Mary University of London. There are limitations for studies like this, such as population bias, since women on menopausal hormone therapy tend to have more money and therefore be healthier overall.
Your gut
A 2024 peer-reviewed study published in the medical journal BMC Medicine found that, after hormone therapy, both trans men and trans women experience changes in gut microbial diversity. Bacteria that help fiber digestion and immune health are more abundant depending on how much testosterone or estrogen is in the body, regardless of sex at birth, the study found. Escherichia coli (which trains the immune system) is more abundant in trans men after hormone therapy, while Coprococcus (which aids digestion) is more abundant in trans women on hormones.
These changes align with gender identity, creating similar gut bacteria for trans and cis men, as well as for trans and cis women.
Estrogen may also lessen the risk of colorectal cancer for menopausal women, according to the Fred Hutchinson Cancer Center. A large-scale study published in 2024, which included more than 28,000 post-menopausal women, found a consistently lower risk of colorectal cancer among those on hormone therapy. Learn more about the links between menopausal hormone therapy and cancer risk from the American Cancer Society.
Your bones
As estrogen declines in menopause, women are at a greater risk of fractures because they are more likely to have low bone mineral density. Hormone therapy, combined with exercise, can offer protection. Studies have shown that taking estrogen and progesterone for bone health is more effective than estrogen only.
Your heart
Testosterone increases red blood cell count in trans men to the same average level of a cis man. But multiple studies, the latest published in 2025, have also found that electrical signals moving through the heart, which make it beat, are also changed by hormone therapy.
Several studies have found that the QTc interval (how long it takes the heart’s chambers to contract) is longer for transfeminine people on estrogen and shorter for transmasculine people on testosterone. These changes also align with gender identity.
Your senses
Estrogen and testosterone affect touch, taste and scent.
Women, both cis and trans, are generally more sensitive to changes in temperature than men. One study published in February found that trans women taking estradiol and cyproterone acetate, a prescription that lowers testosterone levels and mimics progesterone, noticed improved temperature perception on par with cis women.
For menopausal women, hot flashes and night sweats are common reasons to seek out hormone therapy; and there is a robust body of research showing that hormone therapy reduces these symptoms. Menopause can also make certain clothing textures or touches feel painful or more extreme. More research is needed on how hormone therapy can help, but there is some evidence that it can improve collagen production and skin elasticity.
After hormone therapy, transmasculine people may notice thicker and oily skin that’s more prone to acne and sweat, while trans women often become more prone to bruising after hormone therapy and may notice softer skin. Body odor frequently changes for trans people taking hormones, too.
Your brain
A study from 2019 found that, for trans women who have had their testes removed, higher estradiol levels caused by hormone therapy strengthens connections between areas of the brain involved in fine motor skills, learning, emotions and sensory perception.
Research is still uncovering the extent to which estrogen hormone therapy impacts brain health for menopausal women. Brain scans for premenopausal and postmenopausal cis women look different because estrogen receptors become more dense after menopause. A 2024 study into that phenomenon found that for postmenopausal women, higher density in the thalamus (the brain’s information relay station) was associated with depression. Greater density in the hippocampus (linked to learning and memory) was linked to lower scores on some cognitive tests.
These are initial findings, and the first study of its kind. More research is needed to understand how estrogen decline impacts the brain.
(Emily Scherer for The 19th)
Your muscles
Testosterone makes it easier to build muscle and, particularly in trans people, redistributes fat from the hips and thighs to the abdomen. For trans women, estrogen decreases muscle mass over time and causes fat redistribution to the hips and butt.
The loss of estrogen brought on by menopause causes women to lose muscle faster, which makes strength training even more important as you age. Hormone therapy can help, but lifestyle choices are still critical.
Your hair
For trans men and nonbinary people on high doses of testosterone, thick and coarse body hair is a key (and often desired) effect of hormone therapy. So is facial hair that can become a full beard. It often comes, though, with hair thinning at the temples or baldness, which is also dependent on genetics. These changes also occur for cisgender men on testosterone therapy, although they may be less noticeable.
Meanwhile, trans women on hormone therapy can actually see hair regrowth through taking estradiol and antiandrogens (testosterone blockers).
Your sex life
Testosterone ramps up sex drive in those taking it, regardless of gender identity. Estrogen taken through hormone therapy can also decrease sex drive for trans women. Transmasculine people on high doses of T can expect clitoral growth and increased sensitivity. Cis men on testosterone can expect lower sperm production.
Transmasculine people who have been on hormone therapy for at least one year and menopausal cis women experience some of the same changes in their bodies, including vaginal dryness and a similar vaginal microbiome, according to a 2024 study. This change can put trans men at a higher risk for sexually transmitted infections. However, in both cases, localized estradiol cream can help.
Another study published last year found that, for trans women who are taking estrogen through hormone therapy as well as a testosterone blocker, testes looked almost as they did before puberty. The voluntarily donated tissue was examined after the women had orchiectomies, a surgery that removes testicles.
Your sleep
Hormone therapy can help reduce night sweats and hot flashes, which can in turn improve sleep. For cis men, testosterone therapy has the potential to make sleep apnea worse.
One 2024 study analyzed the sleep patterns of nearly 100 trans people. After three months of hormone therapy, small changes in sleep patterns emerged: The sleep midpoint shifted roughly 20 minutes later for transmasculine people, and 20 minutes earlier for transfeminine people. This change aligns with gender identity as well.
In 2023, a study of 73 trans people found that, after three months of hormone therapy, trans men spend less time in deep sleep and reach REM sleep faster.
Ev Nichols, a postdoctoral scholar at the University of California, Los Angeles who studies developmental biology, writes a newsletter where she tracks studies like these as they publish. As a trans woman and a researcher, she is fascinated by how hormones cause previously unknown changes to the body. Doctors need to use this research to provide better care to their patients, she said; and these studies also have political implications.
“One of the things the Trump administration has done is to replace the concept of gender and gender identity with sex, and thinking that sex is this binary, immutable thing,” she said. “Sex is much more complicated than that.”
Enjoyed this article? Sign up for our newsletter to receive regular insights and stay connected.