Thyroid UK
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If I reduce my dose of levothyroxine how soon will it show up in a blood test

Hi, I was due to have an operation this week but it was cancelled on Friday as the hospital said my FT4 level was too high at 17.4 (range 7.8 - 14.4). I spoke to my GP who is in a different NHS trust and she requested a blood test, the results of which were TSH 0.20 (range 0.35 - 5.5) FT4 18.4 (range 9 - 22.7). GP said that although TSH was low as I am feeling ok on a dose of 125 then she didn't see why the hospital would have a problem.

Anyway, my consultant has pushed for my op to be rescheduled as he is pretty miffed that the anaethetist cancelled it and I have been booked in for 14th May on the understanding that I reduce my FT4 and increase my TSH to keep the anaethetist happy!

I reduced my levothyroxine dose to 100 last Friday and am due to have a pre op blood test for the operation on 7th May. Will the reduction in my dose show up in a blood test just over 4 weeks later?? I'm not sure how long a change of dose takes to work.

3 Replies

I think it usually takes about 6 weeks for the TSH to fully reflect a dosage change, but I would have thought that most of the effect will have occurred after 4 weeks.

I would be concerned though that you will start to feel ill with this reduction, as you look to have been well-medicated before. Your GP sounds very sensible by the way. Xx


Thanks Clarebear, to be honest I'm really annoyed with the anaethetist as he is going against the advice from my GP. I was diagnosed 7 years ago and have been stable and functioning pretty well on dose of 125 for last 4 years. I really don't want to mess with the dose, but unless I do the hospital won't operate!! I'll play the game and hope they like my blood test results in 4 weeks on a reduced dose and then as soon as I've had my op I'll go back to the dose I KNOW I feel ok on. Can't believe I'm going to have to cope with the underactive symptoms just to suit some jobs worth at the hospital!!! And yes I've always been happy with the support from my doctors practice, nice to know I've got a good one!!


Yes very sensible. Good luck with the op. xx


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