Which is the most potent estrogen?

• A. Estradiol
• B. Estrone
• C. Estriol
• D. Progesterone

Answer: (A)

Estrogens differ in how strongly they activate estrogen receptors, which determines their potency. The most potent natural estrogen is 17β-estradiol because it binds both estrogen receptor types (ERα and ERβ) with the highest affinity and triggers the strongest estrogenic responses in target tissues such as the uterus, vagina, breast, and bone. This means it produces the greatest biological effects per unit amount.

Estrone has lower receptor affinity and a weaker effect, especially after menopause when it becomes the more circulating form but is less potent. Estriol is the weakest estrogen and mainly reflects pregnancy-related physiology with comparatively limited estrogenic activity. Progesterone is not an estrogen at all; it acts on progesterone receptors and modulates estrogen effects in various tissues, so it’s not considered when ranking estrogens by potency.

So, the strongest estrogen among these is estradiol because of its highest receptor binding and corresponding potent estrogenic action.