Estrogens are important in the development and maintenance of the female sex organs and secondary sex characteristics. They also regulate the menstrual cycle and are important in the maintenance of pregnancy. Estradiol is the most potent naturally occurring estrogen. Estradiol is normally metabolized into estrone, which in turn can be metabolized along two alternative pathways eventually to either estriol or 2-methoxyestrone. The direction of estradiol metabolism is altered in hyperthyroidism and anorexia nervosa in increased formation of 2-hydroxyestrone and subsequently 2-methoxyestrone. This causes a consequent decrease in the formation of estriol.