oestrogen inhibitors & thyroid medication

Hi, I am currently on the 5 years of Anastrozole, which started in September 2013. I had been diagnosed with an Underactive Thyroid 5 years previous and taking Levothyroxine, the synthetic thyroid hormone. Ever since I began taking the both hormones together, I have been on a horrendous journey of severe side effects, which I discovered today are the same in both these hormone drugs. So it feels like I am getting 100 times the side effects all at once! Because the Anastrozole inhibits the production of oestrogen, and the synthetic thyroxine replaces the Thyroid Hormone over again, my body is fighting a losing battle. The worst side effects (most of the side effects from both drugs are the same so you can see why I am struggling!), are irregular and ectopic heartbeats (I had every heart test going and my heart itself is fine), crippling joint, muscle and joint pain, brain fog, fatigue, dry skin and hair, the list goes on! I joined the Thyroid UK forum on FB and it is an invaluable source of information, and I have finally been chatting to others who are in the same situation with having Breast Cancer treatment alongside Thyroid treatment. Does anyone else identify with this and if so, what happened for you? Cheers, Michele. Xxx. ??

Hi Michele, I’m on I’ve been in Levothyroxine for 10 years (100 - 150 mg) and Tamoxifen for nearly 3, I have to say I’ve never had any side effects from my thyroid medication and very few from Tamoxifen but interested to hear of a facebook group. Sorry you’re having such a tough time x

Well, I finally got some answers from the Breast Surgeon. He’s taken me off Anastrozole and increased my Thyroxine until I see him in August. 3weeks yesterday and I have noticed changes in my heart and the painful joints. If he says to restart it I don’t know what to do. The only other choices are change to another inhibitor or stop altogether. ???

Hi Lucy12 and Waveylocks, Thanks both of you for helping me with helpful info. My Levo has been raised from 75 to 100mcg daily, but as I have been taken off Anastrozole until I see the Breast Surgeon I don’t know what happens next in August when my appt is.

Lucy, I get the same s/e too, the exhaustion takes over and it’s an effort to do things especially when everything hurts! As for the irregular heartbeats, they’re still there. And the weight gain! You mentioned STTM book, my local library managed to buy in a copy and I find it interesting. 

Waveylocks, I keep seeing NDT, mentioned in various forums, not all doctors will prescribe it will they? If it’s pricey I doubt I will be so lucky. It was never mentioned to me about the possible link between thyroid meds and AI’s, I found this out from talking to one of the Admin team on Thyroid UK on the Health Unlocked website. They gave me all the help I needed to get something done. My Thyroid Antibodies came back borderline to high, so when they’re done next week I will know if it could be autoimmune.

I notice you both mention Allred score, what’s that? I know I’m oestrogen receptive, but don’t know any numbers.

Hope you are both keeping well. Cheers, Michele ?

I have been reading this thread with great interest! It seems I have just tolerated a lit during the past 5 years blamed myself for being tired, tried to carry on through aches and pains, and not made any links with my meds and neither have the doctors! FUMING

I have been on Levothyroxine since 2009 (other than gaps during radioactive iodine treatment) following diagnosis of thyroid cancer and complete thyroid removal. Breast cancer diagnosis end of 2010 Her2+DCIS so had surgery (lumpectomy, chemo, radiotherapy, herceptin, and hormone therapy) due to finish taking exemestane end if this month. I have had joint pains like you wouldn’t believe, some days I can barely walk upstairs and I just didn’t even think about the tablets, also soooo tired so often and just felt guilty!


Hi Michelle,

so sorry to hear about what has happened & how poorly you are feeling. The most common thing that happens is that people are under treated for hypothyroidism … This causes palpitations & fatigue. No doubt you’ve discovered that you ft4 needs to be near the top of the range. The trouble with levothyroxine is that it is a synthetic hormone & t4 is the storage form so the body has to convert it to t3. Not everyone does this well so you end up with ongoing symptoms… Levothyroxine does not work for everyone -certainly did not for me so the NDT I’m on has all the thyroid hormones in it. 


Ive got to say that since I’ve been on Letrozole it has NOT affected my uptake of thyroid hormone. Sine the other treatments have stopped I’ve been able to lower my dose back down. Been on Letrozole for a year now. Am starting to get some hot flushes at night though…

Do get your vit D level checked via blood test -GP can do it. You need to be around 80 at least. Low vit D causes bone pain, fatigue and un refreshed sleep. I felt awful and so tired could barely walk when mine was very low at 3, a few years back (prior to my cancer diagnosis).  So worth checking. 


Having said all of that my heart does funny little races -am under a cardiologist as apparently the Herceptin caused some damage. So on BP & beta blocker tablets. I feel better on the tablets -felt very tired and weak before…lol


So there’s a fair few factors that could be causing symptoms which are worth checking out before accepting it’s the Letrozole. 


Ihope this helps…and hoping you find a way to ease your symptoms…

hugs xxx 

Hi folks. Just found this forem after I Googled Letrozole and NDT! So already a member of Thyroid Uk. Started on Letrozole in October and take that before bed and all my NDT in the morning. Found it rough to start with butvthinfs have settled down a bit since. Don’t know what my score is, they didn’t tell me divtgsts on my list to ask! Didn’t have to have Chemo but did have radio therapy so. Even interesting reading your experiences. My thyroid state I think is pretty good and I take shedloads of supplements! So no real questions at the moment but just wanted to say hello!

Hi all
I’ve been reading msg in this thread, I have hyperthyroidism and been on carbimazole since Xmas. Started on 20mg now i take 5mg once a day.
I have been on letrozole for 3 weeks and have my radiotherapy planning Oct 2nd.
I am having lots of muscle and joint pains these have Increased since taking letrozole as I was managing the pain on carb.
Off to docs Monday will get bloods done as I don’t see Endo for another month and would like to see if there’s any changes.
I’m taking 2 painkillers at night Naproxen using muscle rub cream for now seems to help.

Is anyone taking anastrozole and Levothyroxine (for an under active thyroid) at the same time , just wonder when was the best time to take the drug?I am also taking isonic acid which I take first thing in the morning and nothing to eat after that. Thanks?

Thanks for the reply yes that sounds like a good plan, I also take supplements so lots of tablets , yes life was much easier before BC

Hi all - I’ve been taking Levothyroxine for underactive thyroid since i was 20 and when i raised the likely side effects of thyroxine and tamoxifen with the onc he advised me to have regular blood tests as the tamoxifen can inhibit the absorbtion of thyroxine (apparently oestrogen is important in the process). So i wasn’t suprised that after 6 months of tamoxifen my thyroxine dose was increased. I’ve slightly altered my morning pill taking routine to ensure that the first pill of the day is the thyroxine on an empty stomach and with water and try not to eat for another hour. The last pill of the morning (before rushing out of the door) is the tamoxifen - so far (2 years in!) I’ve only forgotten a handful of times so have a stash in my desk drawer.

The most recent blood test showed that my thyroid levels are satisfactory using this approach but i’m keeping an eye out for signs of becomming underactive again…

Thanks Belford Belle for the reply, I used to take my tablet first thing in the morning and everything was fine now I have to take the Bisphosphonate so can’t take it then any more ? hopefully my GP will tell me what to do x

Hi Michele

I realise your initial post was over 6 years ago, but i just want to say thanks for posting.  I have been reading through all the comments which has really helped me this morning. 

I have been taking levothyroxine for 11 year, and, for the last 5 years approximately i have suffered with salivary gland pain (when drinking fruity drinks) and a dry mouth with difficulty swallowing - causing post nasal drip at night and a cough in the mornings. 

i was diagnosed with BC in september and had a lumpectomy, started taking anastrozole 1 month ago and have started my radiotherapy this week. 

I have finally (through talking to dentist) seen a specialist about dry mouth and 2 weeks ago was diagnosed with sjogrens syndrome.  Another autoimmune issue to add to hypothyroidism and Coeliac disease.

My dry mouth has got much worse and i was associating it with the Sjogrens, but upon reading all these comment realise it could be a clash between - levothyroxine and anastrozole.

will definitley get this checkout out in the new year once my radiotherapy is finished.

thanks again,  and it would be interesting to hear how everyone is doing after years of treatment