Everyone produces estrogen. However, females have higher levels of estrogen than males. Estrogen affects sexual reproductive development. But, according to the Cleveland Clinic, estrogen plays a role in several functions, including: Estrogen levels may decrease for a variety of reasons. One of the most common causes of low estrogen is menopause. Certain medical conditions and side effects from medication may also lead to a decline in estrogen. When estrogen levels decline, it can lead to symptoms, such as hot flashes, infertility, headaches and sleep and mood changes. Hormone replacement...