What happens when oestrogen is too low or too high, and the parts of the body most affected. The chapter describes how low levels can bring fatigue, mood swings and brittle bones, and why the risks of osteoporosis and heart problems may increase after menopause. It notes that some conditions, such as thyroid disorders, can raise oestrogen, and that prolonged high levels may be linked to conditions like endometriosis. Separate pages explain oestrogen's role in bone growth and turnover, in keeping cholesterol and blood vessels healthy, and in brain chemistry, including its connection with serotonin and the mood changes that fluctuations can cause.