Thyroid pill significantly worsening BC survival?

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?

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.

I’m in the decision phase of dealing with hypothyroidism. My TSH went from always normal to 6.9 this last blood test. I have a repeated blood test in 4 weeks. Found these articles (below) as I’ve searched for issues and comorbidities with thyroid replacement therapy THRT.

They seem to be saying that about 1/3 of women on THRT get breast cancer. That ER+PR+ early node-negative BC (me) have a lower DFS with THRT even with tamoxifen. Sigh.

With the above comments, I’m going to keep looking for research on whether well managed THRT is the difference between good/poor cancer outcomes. Let’s hope so!!!!

Meanwhile, I have a cancer center survivorship appt in 10 days - my provider now is an internist (previously an NP) so she may have good info.
I also see my local endocrinologist in May - she might be helpful - although she has seemed to be resistant to say anything about my health that may be breast cancer related. She defers to the cancer center. It’s frustrating because the cancer center has made it clear that they expect the primary care, or other specialist, to continue to problem solve things like headaches, cholesterol, fatigue, to eliminate causes that might be from regular health issues unrelated to cancer. Seems reasonable to me. But I don’t think the endocrinologist agrees. I may need to change doctors.

THRT association with BC
https://aacrjournals.org/clincancerres/article/27/2/585/83391/Exogenous-Thyroid-Hormone-Is-Associated-with
Hormone+ BC and THRT, w/o Tamoxifen, mentions AI briefly
Hormone- BC and THRT = tamoxifen resistance
https://pubmed.ncbi.nlm.nih.gov/37723865/

@triciaot Thank you much for the helpful links. I completely agree about the treatment gap - I'm feeling the same way even tho I'm only in the first year post diagnosis. I've pushed both my endo and BC onco about these studies and was told that its been known for about a decade that anyone on levothyroxine MIGHT be at higher risk of solid cancers and have higher chance of cancer recurrence, as well as the recurent cancer being more aggressive. However, they THINK those higher risks are only for patients that are on doses of levothyroxine that push TSH to a very low level (essentially hyperthyroid levels). So they said its best to be on the lowest dose possible. I really wish someone would've mentioned this to me years ago, very worrying. I'm hoping I can at least lower my dose.

@lilacs777 It is very concerning, yet hypothyroidism can cause other issues. It’s like being between a rock and a hard place! I did message my primary doc and she is including the test for Hashimoto’s when they retest my TSH. I’m not sure it will make a huge difference because treatment for autoimmune hypothyroidism is the same. But I figured I might as well know - could be new research shows differing outcomes with each type of disease.

Are you on AI? I would think that might protect you some. I’m on tamoxifen and I think levothyroxine might be a problem.

I tend to be non-compliant on taking meds if I have pretty good reason to question outcomes. When my doc ordered 40 mg of statin, I cut it in half. Six months later, lab showed that 20 mg was enough. Stayed on it for 2 years, then on my own stopped taking it because I really had changed my diet over the past year (also knowing I had an upcoming lab in 6 months). Labs look okay, so no more statin.
When I started 20 mg tamoxifen, I started having headaches 24/7 after about 4 months. I started taking 10 mg knowing I had an upcoming appointment and that research was showing that 5 mg might be enough. I still had headaches on 10, got a brain MRI, and they lowered me to 5 mg. I’ve been okay on that.

I’m pretty sure the doc would start me with 12.5 levothyroxine- even that scares me. I know I’ll be tempted to cut the pill and wait to see what the next lab says. Not sure at what point my doc will get too annoyed with me.
I need to do more reading about levothyroxine doses.

I sometimes think that ignorance would be bliss. But I guess that’s not in my DNA 🙂