Estrogen

Estrogen is a collective term for a group of steroid hormones that produce biological effects through estrogen receptors. The three major naturally occurring estrogens in humans are estradiol (E2), estrone (E1), and estriol (E3). Estradiol has particularly high biological activity at estrogen receptors and is the predominant estrogen during much of the reproductive period before menopause .^1,2

Estrogens are produced from androgen precursors through the enzyme aromatase. Aromatase converts testosterone to estradiol and androstenedione to estrone. The ovaries are a major source of estradiol before menopause, but estrogen synthesis also occurs in other tissues through local conversion of androgens. The relative amounts and sources of the different estrogens vary with sex, age, reproductive stage, and pregnancy .¹

Estrogens exert many of their effects by binding to estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ). These receptors belong to the nuclear receptor family and can regulate gene expression after binding estrogen. Estrogen signaling influences reproductive development and function as well as processes in tissues such as bone, the brain, and metabolic tissues .^1,2

Estrogen and estradiol are therefore not synonymous. Estrogen is the broader term for hormones with estrogenic activity, whereas estradiol is one specific estrogen. In everyday and clinical usage, “estrogen” is sometimes used informally to refer to estradiol, but distinguishing the terms is important because estrone and estriol also act through estrogen receptors .^1,2