Estrones

Estrone is a natural estrogen and one of the three major endogenous estrogens in humans, along with estradiol and estriol. The term often refers specifically to estrone (E1), a steroid hormone with estrogenic activity.

Biosynthesis and forms: Estrone is produced primarily by the aromatization of androgens such as androstenedione (and to a lesser extent testosterone) by the enzyme aromatase. In premenopausal women the ovaries are a major source; after menopause, peripheral tissues, especially adipose tissue, become the dominant site of estrone production. Estrone can be interconverted with estradiol by 17β-hydroxysteroid dehydrogenases. Circulating estrone exists in free form and in conjugated forms such as estrone sulfate and estrone glucuronide.

Biological activity: Estrone binds estrogen receptors ERα and ERβ but with lower affinity and lower potency than estradiol, so its estrogenic effects are generally milder. Its relative contribution to overall estrogen signaling increases after menopause when estradiol levels are greatly reduced.

Physiological and clinical relevance: In postmenopausal women, estrone is the predominant circulating estrogen and contributes to maintenance of bone density and other estrogen-dependent tissues. Abnormally high or low estrone levels can be influenced by obesity, aging, or hormone-secreting tumors and may be assessed in clinical investigations.

Measurement: Estrone is measured in serum or urine, often after hydrolysis of conjugates, using immunoassays or mass spectrometry. Its levels are usually reported alongside other estrogens in hormonal profiling.