Here is a fact that surprises many people: in men, only about 15% of the estradiol in the bloodstream is released directly by the testes [1]. The rest is built elsewhere, from testosterone, in fat, bone and brain. Estrogen is not a female hormone that men happen to carry. It is a signal that both sexes make, need, and can have too little or too much of.
How the body makes estrogen
Estradiol, the most active estrogen, is made from androgens such as testosterone by a single enzyme, aromatase, which is encoded by the gene CYP19A1 [1]. In premenopausal women the ovaries are the main source. But aromatase also works in other tissues, including fat, bone and brain, and this conversion outside the glands is the major source of estrogen in men and, after menopause, in women [1].
That has a practical meaning. Estrogen levels are not set only by the ovaries or testes. They also depend on how much raw material is available and how active aromatase is in your tissues, and individual differences in that activity change with age [1].
What estrogen does in the body
Bone is the clearest example. Aromatase activity and estrogen production are needed for bone growth, for reaching peak bone mass and for normal bone remodeling, and with aging, individual differences in estrogen levels may affect bone loss and fracture risk in both men and women [1].
In women, the fall in estrogen at menopause is linked with a broad set of changes. A 2024 review describes a spectrum that runs from hot flashes, urogenital and mood symptoms during perimenopause to chronic changes in the skeleton, cardiovascular system and brain after menopause [2]. Another 2024 review, in Nature Aging, adds that estrogens help maintain energy balance and metabolic health, although it remains unclear how much of age-related metabolic change is truly caused by falling estrogen rather than by aging itself [3].
What the latest research says
The most informative long-term data on estrogen therapy come from randomized trials. A 2026 Cochrane review pooled 24 trials with 45,660 participants, and about 70% of the data came from two studies: HERS and the Women’s Health Initiative (WHI) [4]. Most participants were postmenopausal American women older than 60 who took oral therapy, so the results do not automatically transfer to younger women or to non-oral forms.
In the WHI arm for women with a uterus (16,608 women, about 5.6 years), estrogen plus a progestogen probably made little to no difference to coronary events (risk ratio 1.17, 95% CI 0.95 to 1.44). It probably increased breast cancer (RR 1.27, 1.03 to 1.56) and probably reduced clinical fractures (RR 0.78, 0.71 to 0.86) [4].
In the estrogen-alone arm for women who had had a hysterectomy (10,739 women, about seven years), coronary events (RR 0.94) and breast cancer (RR 0.79, 0.61 to 1.01) probably did not differ meaningfully from placebo, while stroke was probably higher (RR 1.33, 1.06 to 1.67) and fractures lower (RR 0.73, 0.65 to 0.80) [4]. A 2026 review in Obstetrics & Gynecology points to a meta-analysis of 10 randomized trials in 14,282 women in which breast cancer was diagnosed in 3.6% on estrogen alone versus 4.7% on placebo (RR 0.77, 0.65 to 0.91), and it calls for shared decision-making [5].
Men have been studied far less. One small randomized trial gave estradiol gel or placebo for six months to 78 men on androgen deprivation therapy for prostate cancer, whose testosterone is medically suppressed. Estradiol did not significantly change total fat mass (an average difference of about 1 kg, P = 0.09), although android fat did rise (P = 0.04) [6]. The authors called the result suggestive only: the trial closed early because of the COVID pandemic, and it cannot tell us what happens in healthy men.
What this means in practice
Estrogen is neither good nor bad. Too little is linked with bone loss and menopausal symptoms, while added estrogen carries real, measurable risks that depend on the type, the dose, the route, a woman’s age and whether she has a uterus [4][5]. For men, estradiol is part of normal bone biology [1], so the idea of pushing it as low as possible deserves caution. Whether any of this applies to you is a question for a clinician who knows your history.
Key takeaways
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Estrogen is made from androgens by aromatase in both sexes. In men, most estradiol comes from conversion outside the testes [1].
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Estrogen supports bone in men and women, and its fall at menopause is tied to a wide range of symptoms [1][2].
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In large trials, hormone therapy lowered fractures but raised stroke risk, and combined therapy also raised breast cancer risk [4].
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The trial evidence comes mostly from older women on oral therapy, so personal decisions need a clinician who knows your history [4][5].
Coming next: Day 3 looks at progesterone, the half of the story that often gets forgotten.
References
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Gennari L, Merlotti D, Nuti R. Aromatase activity and bone loss. Adv Clin Chem. 2011;54:129-164. PMID 21874760
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Yang JL, et al. Estrogen deficiency in the menopause and the role of hormone therapy: integrating the findings of basic science research with clinical trials. Menopause. 2024.
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Camon C, et al. Exploring the effects of estrogen deficiency and aging on organismal homeostasis during menopause. Nature Aging. 2024.
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Bofill Rodriguez M, Yong LN, Mirkov S, Bekos C, Lethaby A, Farquhar C. Long-term hormone therapy for perimenopausal and postmenopausal women. Cochrane Database Syst Rev. 2026;9:CD004143. PMID 42695385
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Kaunitz AM, Wright JD. Menopausal estrogen therapy and risk of breast cancer. Obstet Gynecol. 2026;148(5):584-590. PMID 42594383
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Russell N, Hoermann R, Cheung AS, et al. Effects of estradiol on fat in men undergoing androgen deprivation therapy: a randomized trial. Eur J Endocrinol. 2021;186(1):9-23. PMID 34678758
This post is for general education only and is not medical advice. Talk to a qualified clinician about your own hormone health.