Thyroid pill significantly worsening BC survival?

Posted by lilacs777 @lilacs777, Nov 3, 2024

I take levothyroxine due to hypothyroidism and just read this very alarming study that levothyroxine / thyroid hormones INCREASE estrogen receptors and ER+ BC growth! This is very alarming considering the women in this study were all Stage 1 node negative but had very high recurrence and death rates. https://www.thyroid.org/patient-thyroid-information/ct-for-patients/october-2022/vol-15-issue-10-p-7-8/
Has anyone heard of this connection or been told about this by their onco? I'm going to ask about it at my next appt but I'm really worried since when I read more about this they said its been known for a long time that thyroid hormone acts just like estrogen for breast cancer growth.

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windyshores | @windyshores | Nov 3, 2024

I found that article confusing. It seems to be saying that as long as thyroid meds keep levels in the normal range, you are okay. We on this forum are not a professional obviously so I would ask your doctor about this study. It certainly argues for more stringent monitoring of thyroid levels.

It talks about DFS, disease free survival, in relatively short durations so I was not sure if this was after recurrence or at diagnosis.

Finally, taking aromatase inhibitors eliminates estrogen in the body (the remaining estrogen from adrenals) or at least below detectable levels. The article only mentions tamoxifen.

Is this a chicken and egg kind of thing, with cancer before thyroid or thyroid before cancer issues?

Miriam, Volunteer Mentor | @mir123 | Nov 3, 2024

Like windyshores, I had some trouble understanding the article.
This one from NIH was easier for me -- https://pmc.ncbi.nlm.nih.gov/articles/PMC7882351/

It states--"This research demonstrated that hyperthyroidism, AITD, and thyroid cancer are significantly associated with an increased risk of BC, while hypothyroidism can reduce the risk of BC."

I have low thyroid and have been on levothyroxine for 30 years. Without it I doubt I could function. Both low and high thyroid can be life threatening, but thankfully both are treatable. I got breast cancer 2 years ago--decades after I started replacement. Of course it can be worrisome to read this kind of association, but it looks like as long as thyroid replacement stays in normal zone there is no risk. By all means talk to your PCP or specialist--I'm just reading as an interested patient, no expert in any way. But as a fellow patient I'd encourage you not to worry too much. Most breast cancer has no known cause. If you do talk to a doctor, by all means fill us in! I'd be interested to know more.