Oestradiol is a steroid hormone that is primarily produced in the ovaries of women and to a lesser extent in the testes of men. It is one of three types of estrogen and has important functions in regulating the female reproductive system, as well as promoting breast tissue growth and maintaining bone density. Oestradiol levels naturally fluctuate throughout the menstrual cycle, peaking at ovulation in pre-menopausal women. As women age, their oestradiol levels decrease and eventually stop altogether with menopause. Low levels of oestradiol can lead to symptoms commonly associated with menopause such as hot flushes, night sweats, and mood swings. Additionally, low levels of oestradiol can increase the risk of developing osteoporosis.