Estradiol less than 5 pmol/L belongs to the low level of estrogen, which may be caused by being in menopause, premature ovarian failure, polycystic ovary syndrome and so on. 1. Menopause: When a woman is in menopause, ovarian function declines and estrogen secretion is insufficient, which may result in estradiol less than 5 pmol/L. 2. Premature Ovarian Failure: If a woman is under the age of 40 and her ovarian function has declined before she reaches menopause, it means that premature ovarian failure has occurred, which may also make the estradiol level less than 5 pmol/L. 3. Polycystic Ovary Syndrome (PCOS): If a woman has elevated androgen levels in addition to lowered estrogen levels, symptoms such as irregular menstruation and excessive hairiness indicate the presence of polycystic ovary syndrome (PCOS), which may cause estradiol levels of less than 5 pmol/L. When a woman has a test result of estradiol less than 5 pmol/L, she needs to consult a doctor in a timely manner.