Oestradiol is a female steroid hormone produced in women's ovaries and, to a much lesser extent, in men's testes. It accounts for 80% of the oestrogen in the female body and is responsible for developing and maintaining the reproductive system, including fat distribution, preparing the follicles in the ovary for the release of an egg, and helping maintain bone density.
Men naturally convert some testosterone to oestradiol, and it's important for bone mass and sexual function. However, sometimes men can produce too much oestradiol compared with testosterone. This can result in female characteristics, especially the form of breast tissue (gynecomastia, moobs, or man-boobs).