Estrogen is a hormone that is produced by a woman's ovaries. Throughout your life, your estrogen levels will rise and fall. Hair growth accelerates as estrogen levels increase, while later in life you may experience hair loss as estrogen levels drop.
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Rising Estrogen and Hair Growth
When a girl enters into puberty, normally between the ages of 11 and 14 years old, her estrogen levels begin to rise in order to manage her reproductive system. One of the effects of this increase in estrogen is the appearance of pubic and underarm hair, known as secondary sex characteristics.
Decline of Estrogen
A decline of estrogen, whether due to menopause or other hormonal imbalances, can also affect hair growth. You'll experience a thinning or loss of pubic hair as well as hair on your scalp if you have low levels of estrogen in your body. You might also experience unwanted hair growth on your face during menopause, when your estrogen levels are at their lowest. This phenomenon occurs because the lack of estrogen leaves you with a hormonal imbalance of sorts; you have more androgens, or male hormones, than female hormones in your body, which contributes to some male-like symptoms such as body and facial hair.
Hormone Replacement Therapy
Taking estrogen supplements as part of a hormone replacement therapy regime has its effects on your hair as well. Women can treat a type of hair loss called androgenic alopecia, associated with low estrogen and progesterone, by replacing these hormones artificially. Dosages of synthetic estrogen should be monitored regularly -- abnormal hair growth is a possible side effect of one kind of hormone replacement therapy, called esterified estrogens.