Estradiol is the Predominant Estrogen in Adult Men
Estradiol is the main biologically active estrogen in men, circulating at concentrations of 2–3 ng/dL (approximately 20–30 pg/mL), which actually exceeds the levels found in postmenopausal women. 1, 2
Estrogen Production in Men
Approximately 80% of circulating estradiol in men is produced through peripheral aromatization of testosterone, primarily in adipose tissue, with only 20% directly secreted by the testes 1
The daily production rate of estradiol in men is 25–40 micrograms per 24 hours, which is significantly higher than in postmenopausal women 1
Aromatase enzyme, which converts testosterone to estradiol, is abundantly expressed in Leydig cells, seminiferous epithelium, brain, penis, adipose tissue, and other male organs 2, 3
Why Estradiol Dominates Over Other Estrogens
Estrone is the other major estrogen in men, originating from aromatization of adrenal androstenedione, but only about 5% of plasma estrone is converted to estradiol 1
Estradiol concentrations in rete testis fluid can reach levels normally found only in females, and in some species, nanomolar concentrations of estrone sulfate are found in semen 3
Testosterone serves as the major source of plasma estradiol in men, making estradiol the predominant biologically active estrogen rather than estrone or estriol 1, 4
Age-Related Considerations
Despite age-associated decreases in testosterone levels, plasma estradiol levels remain relatively stable in aging men due to increasing aromatase activity with age and increased fat mass 1
Free and bioavailable estradiol levels decrease only modestly with age, while the ratio of free testosterone to free estradiol decreases, reflecting increased aromatization of testosterone in older men 1
Clinical Significance
Estradiol in men is essential for modulating libido, erectile function, spermatogenesis, bone health, gonadotropin feedback regulation, and lipid metabolism 2, 4
Estrogen receptors (ESR1 and ESR2) are present throughout the male reproductive tract, including testis, efferent ductules, epididymis, brain, penis (particularly around neurovascular bundles), bone, and adipose tissue 2, 3, 5
Low testosterone combined with elevated estrogen increases the incidence of erectile dysfunction independently of one another 2