Detailed definition
Estrone, abbreviated E1, is one of the three endogenous human estrogens. Before menopause it is a minor estrogen, with estradiol dominating. After menopause, when ovarian estradiol production stops, estrone becomes the most abundant circulating estrogen, produced primarily by the aromatization of adrenal androstenedione in adipose tissue, liver, and skin. Estrone binds estrogen receptors with much weaker affinity than estradiol — its relative potency at the estrogen receptor is roughly 10–15% that of estradiol — and it is interconverted with estradiol by 17β-hydroxysteroid dehydrogenase. Postmenopausal serum estrone levels typically range from 30 to 70 pg/mL, depending heavily on body fat. The conjugated equine estrogens in Premarin contain estrone sulfate as a major component. Some compounded "Bi-est" preparations also contain estrone, although the dominant compounded combinations in modern practice are estradiol and estriol.
Why it matters in menopause
Although postmenopausal estrone levels can look reasonable on a blood test, especially in women with higher body fat, estrone alone does not relieve menopausal symptoms or protect bone the way estradiol does. This is why simply having "some estrogen" after menopause does not prevent hot flashes, GSM, or accelerated bone loss. Treatment uses estradiol because that is the molecule the body actually used during the reproductive decades.