Thyroid UK
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Blood testing for vitamins

Hi All - I'm considering having some blood tests for eg. iron, D3 and B12. I believe that you are supposed to stop taking the relevant supplements, at some point before the tests. However, wouldn't it be more useful to know whether the supplements I am taking are doing the job?? Therefore, surely it would be better to CONTINUE taking the supplement?

3 Replies

The problem is that when you are taking the supplements there is often extra in the blood as you are trying to increase your stores of the vitamins especially if you have recently taken a tablet. I'm not sure of the length of time you should be off the supplements though.

I suppose it's similar to not taking thyroxine, or whatever thyroid hormone you take, right before a blood test. Blood levels will be temporarily higher than usual shortly after a dose thereby giving a false impression of what is actually happening.


Chooselife, you want to know the levels of vitamins circulating in your blood not the peak levels after ingesting a supplement so for most things leaving 24 hours between last dose and blood draw is sufficient. I believe that iron should be stopped a week or two prior to testing but I don't have a link to back up the assertion and if you want a baseline result for B12 to show your levels without supplementing you need to stop 8 weeks prior to the blood test.


Hi CarolynB and Clutter - thanks so much for responding. I was feeling very puzzled about this, as was thinking I need to know if my supplements are doing the job or not and should therefore have the blood tests done while still taking supplements. However, I think you are right - I need to stop the supplements beforehand.

As the severity of my thyroid/adrenal issues mean that I am housebound, I'm going to try the fingerprick home testing service that Blue Horizon do (which, bizarrely isn't mentioned on their homepage - I found out about it via STTM).

Thanks again for responding. It's wonderful to not feel so alone :) .


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