Is Tamoxifen working? Endoxifen Blood Test Numbers are Off!

BCAE

I'm on my 4th year of Tamoxifen. I posted previously that my MO did a hormone test (including estradiol) to determine if I was pre-menopausal. Based upon the results and the concerns voiced by my Ob/Gyn that my estradiol numbers were too high, my MO suggested we try lowering the dose of Tamoxifen from 20mg to 10mg. She then added for me do a blood test for Endoxifen levels to make sure that the Tamoxifen was working at the lower dose. My Endoxifen level ended up being too low. We then decided to increase my dose to 15mg for a month, at the end of which then repeat the Endoxifen level test. I got the exact same result as the previous time! At this point, I found an article that said that higher levels of Curcumin (or Turmeric) could interfere with Tamoxifen effectiveness. Even though I was only on 750mg (which is lower than used in the study), I was taking it with Tamoxifen every night. So I stopped the Curcumin for 6 weeks and then re-tested the Endoxifen levels. This time my test came back even *lower* than the previous 2 times! Huh?! Right now I'm waiting to hear back from my MO about the latest lab results, but I'm really at a loss as why my Endoxifen levels would go down again! After my first Endoxifen test result came back low, my dietician suggested that I do the CYP2D6 enzyme test to see if I'm a bad metabolizer of Tamoxifen, but my MO said that insurance may not cover the test, the test is not so reliable, and that the Endoxifen blood test achieves the same result. I'm wondering if this Endoxifen test isn't so reliable too? Any suggestions on what to try next??

jhl

Hi BCAE,

I agree with your MO the testing for CYP2D6 is not reliable and the interpretation of the test can be difficult:

https://www.nature.com/articles/gim2012108

https://www.mayo.edu/research/forefront/genotyping...

Do you know what your endoxifen level is? I don't believe it should be lower than 5.9 ng/ml.

When I read your description of your journey with tamoxifen, it strikes me that at the beginning, when your estradiol level was high, why were you not put on a luteinizing hormone-releasing hormone which would suppress ovarian function? Tamoxifen is an inducer of ovarian function and can affect the hypothalamic-pituitary axis to prevent the feedback to turn off ovarian function in some women. Increased estradiol levels is not an unexpected consequence of tamoxifen therapy. This article deals with follicular cyst formation but explains the feedback mechanism pretty well:

https://springerplus.springeropen.com/articles/10....

Personally, if it were me, I would go back to your MO and ask about LHRH modulators rather than mess with your tamoxifen dose.

Best of luck,

Jane

BCAE

Yes, my Endoxifen levels tested at 4.5, 4.5, and 1.5. Thanks for sending over the articles about about the test and also about LHRH modulators. I'll take a look..my MO hasn't said anything about the LHRH before, but I just started seeing her. My previous MO never thought to test my estradiol levels.