high estradiol level - tamoxifen

Hello,

My wife is on her fifth year of tamoxifen post- breast cancer (following single lumpectomy, chemo and radiation treatments).  She is almost certainly post-menopausal based on FSH levels taken over time.  Her estradiol tested this week at 933pg/mL.  Her doctor said the reason her estradiol is high is because she's taking tamoxifen.  We're not sure we understand this - isn't tamoxifen supposed to lower estradiol levels?

Thanks for your advice.

[Cleveland Clinic]

Dear peecvee,  Some breast cancer cells require estrogen to grow.  Estrogen binds to and activates the estrogen receptor in these cells. Tamoxifen is metabolized into compounds that also bind to the estrogen receptor but do not activate it. Because of this competitive antagonism, tamoxifen acts like a key broken off in the lock that prevents any other key from being inserted, preventing estrogen from binding to its receptor. Hence breast cancer cell growth is blocked,and levels of estrogen (estradiol) in the blood would be increased.