Estradiol is a steroid hormone produced from testosterone via the aromatase system in the granulosa cells of ovarian follicles. It is instrumental in the development of secondary sex characteristics at puberty and in the menstrual cycle. Plasma levels of estradiol peak during the follicular phase of the menstrual cycle to approximately 300 pg/ml. During this time, it stimulates proliferation of granulosa cells, increases the size of uterine glands, and exerts positive feedback on the hypothalamus, leading to an increase in luteinizing hormone. Blood levels of estradiol drop as luteinizing hormone levels increase and trigger ovulation, then rise again during the luteal phase to approximately 100 pg/ml. Estradiol is metabolized into estrone by 17beta-hydroxysteroid dehydrogenase 2 and hydroxylated metabolites such as estriol, as well as glucuronidated and sulfonated metabolites, which are excreted in the urine and feces.