This blood test checks for unusual levels of testosterone and oestradiol, which may affect sexual libido, erectile function and fertility in men and ‘male characteristics’ in women such as excessive hair or help to explain abnormal periods, heavy bleeding or symptoms of menopause. Oestradiol is a steroid hormone and accounts for 80% of the oestrogen in women. It is secreted by the ovaries and is responsible for the development and maintenance of reproductive structures, including fat distribution, preparing the follicle in the ovary for the release of an egg, getting ready for fertility and helping to maintain bone density. In men, high levels of oestradiol can cause the development of breast tissue (gynaecomastia or man boobs) and affect sexual function.