Carbimazole, rashes/hives.. Is it dangerous?

Hi there

I have been on Carbimazole, 40mg, 3-4 weeks, diagnosed T4:51

The last week or so, I have begun noticing hives/rashes under my forearm here and there, half an hour after taking Carbimazole. These have increased daily, today I experienced a widespread rash over my lower back, again, hive looking, and then 2 hours later, on my inner elbow. I also feel breathless for a while.

Is this dangerous? I am worried that I am having an allergy? Is this common at all?

Should I continue to take the Carbimazole tomorrow?

Any help greatly appreciated


21 Replies

  • Carbimazole can have serious effects:

    Common side-effects tend to be minor, but on occasions, carbimazole can cause serious blood disorders. If you get an infection, a sore throat, mouth ulcers, a high temperature, any unusual bruising, or if you just feel generally unwell, contact your doctor for advice straightaway.


    Can carbimazole cause problems?

    Along with their useful effects, most medicines can cause unwanted side-effects although not everyone experiences them. The table below contains some of the most common ones associated with carbimazole. You will find a full list in the manufacturer's information leaflet supplied with your medicine. The unwanted effects often improve as your body adjusts to the new medicine, but speak with your doctor or pharmacist if any of the following continue or become troublesome.

    Common carbimazole side-effects What can I do if I experience this?

    Feeling sick, stomach upset Stick to simple foods - avoid rich or spicy meals

    Headache, muscle and joint pain Ask your pharmacist to recommend a suitable painkiller

    Skin rash, itching Let your doctor know about this, as treatment with an antihistamine medicine may help

    Changes to the way things taste, hair loss These should pass after the first few weeks

    It is difficult for patients to know whether they are in the "seek help straightaway" or the "ask your doctor" group. However, if you think you are in the straightaway group, the advice goes as far as going to A&E now - not tomorrow. If you have any antihistamine tablets, you could try one.

  • Thank you thats so helpful, I have been to A&E... my rash was widespread rash, absolutely massive welts/hives, will repost as I am not sure what to do!

    Thank you :-)

  • Sounds awful.

    You were right to go to A&E - but if they have not provided you with some relief, I really don't know what to suggest. Obviously, some sort of antihistamine would appear to be an option, but if I can see that, both you and A&E will already have thought of it!

  • I had rashes on my arms when I was on it but the GP I saw wasn't concerned although he did say most doctors would have taken me off it. He just gave me anti histamines. I felt quite unwell on it so the endo changed me to the one that gets abbreviated to ptu's and I was OK on that one.

  • Thank you, my rash is widespread hives, enormous ones over torso, bum and arms :-( I've just been to A&E, so will write a repost. Thank yu for your help :-)

  • Sounds like you need to come off it. Good luck

  • Thank you... Docs said to stay on and take antihistamine :-(

  • Hi Wildpoppy,

    Exactly the same thing happened to me. I contacted the hospital, but my Endo was away and one of his colleagues told me to stop the Carbimazole and get a prescription for PTU off my GP instead, which I did. However I then read that this was very common reaction after being on Carbimazole for a few weeks and it soon passed, so I took antihistamines and carried on with the Carbimazole. Sure enough after another couple of weeks, no problem.

  • Ann!

    Really?! May I ask how bad the rash was?

    I have just reposted giving more details... But my rash is quite severe, over all torso, chest, neck ears, back, bum, few bits and bobs on my legs... I am back from A&E as it got really bad... They gave me Piriton, which has taken 70% away, but they are still appearing... Really itchy, red & white, hot welts.

    I take 40mg of Carbimazole all in the morning, do you think it would help to split the dose to half morning, half evening?

    Did the antihistamines stop yours completely immediately or did it take a couple of days?

    I really am scared of carrying on with the Carbi tomorrow :-( especially as the antihistamines have not yet seems to get rid of them... I have only had one dose so far though?

    Really appreciate any help...

  • I'm so sorry you're in so much distress. I hope it eases very soon.

    My experience was about seven years ago and although I still remember how dreadful it was (like you hives all over my body) I can't remember exactly how long ot took to subside. I think that once I started taking the antihistamine it started to calm down pretty quickly.

    If you are really worried or the antihistamines aren't as affective as they were with me, you could ask your doctor the prescribe PTU (propylthiouracil - even the doctors can't pronouns it) instead for a while. The main problem with that is you have to take it several times a day, which can be a bit of a nuisance, and then try Carbimazole again later. I don't know if dividing your dose of Carbimazole would make much difference, but it's another option you could try.

    I hope you feel much better soon.

  • Thank you Ann...Appreciate you taking the time to reply... I am distressed, it's awful, especially as they are not going away... I am due for my next dose of piriton now, and they are coming back thick and fast :-( I will sleep on I think, I feel exhausted... See how I go through the night... Keep your fingers crossed for me... What an awful disease this Hyperthyroidism is turning out to be... I had only just recovered from coeliac!

    If I do take Carbi tomorrow, I will definitely split the dose... Theres no way I can face a full 40mg in the morning :-/

    Yes, PTU... I could try for a bit I guess?

    Again, thank you, sincerely appreciated...:-)

  • My dose was split I think although it was a long time ago now. It may help. I was on block and replace. When I was switched to the PTU I know that was split into little doses and I had no issues on that. I wonder why they don't just change you. Probably a cost thing.

  • Thank you... I will ring the Endo's secretary tomo, see what they say, as I still have 7 weeks until my first appt! I only took half my Carbimazole today (20mg instead of 40mg)... Much better, a few hives here and there, but more manageable. :-) I guess I see what they say... Ugh!

  • I hope you have a good night. The old antihistamines used to make you very sleepy. Unfortunately the new ones don't. This is the time it would have been beneficial to have slept through the worst of it.

  • Thanks Ann... I slept ok, although I set my alarm to check myself! Bit silly, but I am a worrier! I only took half my dose today... Less hives, more manageable... But I doubt they will let me continue on such a low dose...

  • Hi,

    I'm glad you had a good night. With luck the hives won't last too long and you'll be able to increase the dose again. In the meantime half is better than nothing and you'd probably have been able to reduce the dose before too long anyway.

    I wish I could remember how long it was before I could stop taking the antihistamine, but it was a long time ago and my memory is terrible anyway, I don't think it was very long.

    I hope you'll be feeling much better soon.

  • Thanks Ann, you've been so lovely to answer me, really appreciated.

    Yes... I was so frightened of taking it today! So I split the dose, 10mg in morning and 10 this evening. I'm not sure if you're allowed to split it, but I thought it would be better than having zero Carbimazole. I will prob do the same tomorrow, and try to speak to the Endo's secretary, hopefully they will help...

    Again, thank you so much <3 May I ask, are you still on any meds? Sorry if you have told me this previously!! Do you have Graves?

    I am still hoping that one day I will get into remission!

  • Yes I was diagnosed with Graves, though the Endo thought I had nodules at first. I was extremely hyperthyroid.

    I had three courses of treatment using Carbimazole, but each time the course of treatment ended my thyroid hormones went crazy again, so I eventually agreed to RAI, which kills the thyroid so now I'm hypothyroid and have to take replacement hormones.

    My doctor will only prescribe Levothyroxine (T4) so I buy T3 myself, as I'm one of the minority who don't do so well on T4 alone.

  • Oh wow, thats so interesting... 3 courses... and still no remission? Did you try diet at all? I hear that it can help along with selenium...

    The RAI sounds scary, but completely understand why it gets done... Are you able to maintain a good level being hypo? Its rubbish that the docs won't prescribe what you need... Socking that they are so behind with it. I have asked 3 consultants now, they have all said that stress has nothing to do with thyroid! Which is obviously not what I am reading (and also possibly my cause too)...

    Have you ever tried any natural thyroid? I hear it works well?

  • oops, socking? I meant shocking!! (must be passed my bedtime!)

  • Sorry I didn't get back to you last night. I tried to, but my brain had stopped functioning and nothing I wrote was making any sense. After about half an hour of going round in circles, I thought that you'd probably gone to bed anyway, so I decided to start afresh after I'd slept on it.

    Graves, unfortunately, wasn't my first autoimmune illness, so I have tried various diets over the last thirty plus years mainly in an attempt to control IBD. These have included cutting out gluten for a couple of years; cutting out all dairy for a further two years and even a very strict exclusion diet, supervised by the hospital, followed by nasal gastric tube feeding, but I still eventually ended up having to have my colon removed. Because of the re constructive surgery I had following that I now have quite a restrictive diet because of problems with digestion and gut motility.

    I have to have B12 injections, because I have lost the part of the ileum which is needed for normal B12 absorption, and my GP prescribes multivitamins and minerals, plus I add a few myself. I have also tried various herbal supplements, and I'm currently taking holy basil and Rhodiola in an attempt to reduce my high cortisol level. Apparently I may have Cushings syndrome, according to the saliva test. I certainly have all the symptoms. I previously had it years ago, when I was on an extremely high dose of steroids, in an attempt to make me well enough to undergo surgery. Of course, once I was able to stop taking the steroids the Cushings resolved itself. This isn't the cause this time, so I'm hoping that the herbal remedies will work as the NHS treatments don't sound at all nice!

    I have had a dabble with NDT, in fact I have just recently, but it takes a while to adjust (There's no quick fix with this illness. Everything requires a lot of patience) and circumstances arose where I needed to be at my best so I reverted back to my predictable T3. I will probably have another attempt once I get my dose stable again. I have to time these things well away from blood tests at my GPs surgery in case it causes my TSH to not be "in range" as this triggers the 6 weekly blood tests. Now that I know what dose of T4 keeps my TSH “”in range” (after only taking the prescribed dose of T4 for 2-3 years and never feeling well) I make sure that I take that for several weeks prior to the one or two blood tests per year my GP does, so that for the rest of the time I'm left alone to do my own thing, i.e. add T3 or try NDT and adjust the dose to how I feel. Like many others on this forum, I pay for my own blood tests, which include all the ones the NHS don't do, such as Reverse T3 and antibodies, which show why some people continue to feel unwell even when TSH levels are “normal”.

    Basically, it boils down to: a lot of people go into remission after treatment with Carbimazole; a few continue to treat with Carbimazole for years, though doctors don't like that; the remainder choose between RAI and surgery, followed by replacement T4 (Levothyroxine); 80% of those have no problem converting to T3 and do just fine; the rest of us are on this forum helping each other as the NHS doesn't recognise that not everyone is ok on T4 alone or feel great when their TSH is within the “normal” range.

    Hopefully you'll be in the first category, not the last, but please continue to come here for information and support. There's loads of combined experience and some very knowledgeable people. Also nice to hear some success stories, but those people either don't come here looking for help in the first place, or tend to drift away to get on with their lives.

    I hope that you were able to find some useful information amongst all that!

You may also like...