Thyroid UK
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Is it normal develope Hypothyroidism state during medication with Carbimazol drug ?

Hi everybody,

I am on Carbimazol 20mg/day since Jan 7, 2013. My blood results when tested Feb, 6, 2013

free T3 6.08 ( rang 2.8-6.8) free T4 14.5 ( range 12-22) TSH <0.005 ( range .27-4.2).

My ENDO said that I should reduce Carbimazol to 15mg/day and to redo the test in few days

time .My latest results as of today are Free T3 4.42 within normal range , free T4 is 10.34 below normal range ,my TSH is better it is .04 . I do understand what is going on am I going to be hypo ? I am not going to see my ENDO for a couple of two weeks .Has any one gone through this before?

11 Replies

It is all too common!

There are two plans with carbimazole:

Start with enough to get the hyperthyroidism under control, then reduce it so that the thyroid hormone levels are right. Which is difficult to get right and the levels can go off very quickly.

Or take enough to suppress the production of thyroid hormone completely and then supply you with levothyroxine as if you had no thyroid! This is often called block-and-replace.

But we see so many people where these do not work as well as the doctors claim and so often they suffer from becoming hypothyroid.

I hope people with personal experience post - wishing you well.



Thank you , Is it advisable to reduce cabimazole dose ?


That is the question I would have asked if I were you!

Honestly, I do not feel qualified to provide any sort of answer. I end up lamenting the idea that people in your situation do not get a pile of blood test requests and could go and get tests done and see the endo again when you feel the need.

It is, in my view crazy that this is how people are handled.

I'd love to see a few endos put on carbimazole for a few weeks and see what they think after that experience.


Hi you need at least 6 weeks between blood tests to get a true idea as to what is going on.

Has your endo told you if you have Graves or Hashis or if it is temporary toxicosis? This will have a bearing on what happens to your numbers.

It may be that you will have to reduce your carbimazole but do not do so until you have seen your endo. If you are concerned then phone the endos secretary and ask for an earlier appointment or phone consult.

Dont worry too much about the blood results numbers at this stage as everything is subject to change. It took my body months to settle down on the carb. Good luck.


Thank you for your response, My ENDO thinks its Greaves disease, he is hard to find or talk to,to be honest ,with the appointments, testing, and figuring out what T4 3 and TSH mean plus making sense as what ENDO says makes me totally sad and confused. I am an engineer by profession some times I think I why not I take matters in my hand and rely on the guidance of people how went through this path before. Once more thank you.


Hi I have Graves so I do know what you are going through. Do try to educate yourself and then arm yourself with loads of questions and try to get a partner or good friend to go with you when you next see your endo.

Get them to do an antibodies test to try and confirm Graves. Also ask for all your iron levels, vitamin D and B12 to be tested as they are often not the correct levels with us hypers.

Any other questions just ask.


I had the same experience. I went from very high T4 to very low very quickly (about three weeks) so it does happen. Call your GP and ask them to ring back at the first available opportunity to discuss your dose.


Hi Hadi,

We are on the same boat!

I was diagnosed with hyperthyroidsm in Dec 2012 and started carbimazole in Jan 2013.

Lab Range

FT4->9.0 to 25

TSH->0.4 to 4.75








Carbimazole->30mg(GP increased by 5mg said it was still high)








Carbimazole->20mg(I took 20mg as told by GP for 2 weeks before reducing to 10mg for last 2 weeks as FT4 near low range)




GP propose reduce dosage to 5mg however i am already at hypo state so propose to stop medication. Do blood test every 2 weeks to monitor.


How do you tell if you have gone into a hypo state. I know what it feels like to be hyper and have been on 15mg of Carb since Aug 2013 and was wondering whether I should lower my dose. I don't feel particularly tired but I am continuously hungry and have put on a bit of weight?


Who's monitoring you? Do you have an Endo? Regular blood tests etc.? Really need blood tests at least once a month in the early days to establish what's going on. Why don't you get copies of your results and post them as a new question.



Nobody is monitoring me and that is what I found strange. I don't know whether it's because my symptoms settled so quickly after taking carbimazole that my GP figures he's got my thyroid function under control or not, but he simply said come back in 12 months for another blood test! No referral to an Endo and no test results, but, as you suggest, I will ask for a copy of the results and will post them on here and then perhaps someone can interpret them for me. I am even beginning to question whether I really have Graves or whether it was a bit of a glitch with my thyroid. Nobody else's thyroid function seems to have settled as quickly as mine and I have none of the other associated Graves' symptoms, so I am not too sure really. What do you think?


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