We have three major estrogens; estrone or E1, estradiol or E2, an estriol or E3. E1 goes through the liver to be detoxified and metabolized. How it is metabolized is very important and impacts whether one is more at risk for estrogen related cancers or not.
When your ob/gyn tests your estrogen level she is typically only speaking about E2, or estradiol. This is the estrogen predominantly seen from ovulation.
Many peri-menopausal and postmenopausal woman have low levels of this type but might still have high levels of other estrogens.
Fat cells convert cholesterol to estrone (E1), this is not measured on standard tests. E1 can then breakdown into metabolites, some are more powerful than estradiol.
How well estrogen breaks down depends on your liver and the bacteria in your bowel. This is one reason that there are now studies underway looking at the link between anti-biotic use and increased risks for breast cancer.
Anti-biotics alter your bowel flora, typically wreaking havoc.
If your liver is very busy detoxifying alcohol, tylenol and pesticides as well as various other medications such as anti-depressants, it may not detoxify or metabolize your estrogens in a balanced manner and can leave you with a hormone imbalance or at risk for estrogen related cancers like breast cancer.