Estrogen is the primary female hormone, but it is also present in low levels in men. Low estrogen can lead to symptoms such as fatigue, weight gain, depression, and mood swings. In women, it can also cause irregular periods and difficulty conceiving.
Estrogen levels decrease naturally with age as part of the aging process. However, certain medical conditions or treatments may also cause low estrogen levels at any age. Common causes include premature ovarian failure or damage to the ovaries caused by cancer treatment or surgery; conditions that affect hormone production from the pituitary gland; and long-term use of medications like glucocorticoids or aromatase inhibitors for cancer treatment.
1 Here are the 10 well studied symptoms of low estrogen

Breast tenderness
Breast tenderness due to low estrogen is an all too common symptom of menopause. During this time, the body’s natural estrogen levels decline and can cause a variety of uncomfortable symptoms. From mild to severe breast tenderness, women may experience soreness or even lumps in the breasts during menopause.
The most effective way to reduce breast tenderness caused by low estrogen is hormone therapy. This treatment works by supplying the body with a synthetic form of hormones that have similar effects on the body as natural hormones do. Hormone therapy has been shown to relieve many of the uncomfortable symptoms associated with menopause including hot flashes, night sweats, and vaginal dryness as well as breast tenderness due to low estrogen levels.