Take levo before or after blood test?

Not sure if this is allowed but have been reading a post on the Facebook group Thyroid Group UK which someone mentions not taking levo before bloods. I have read on here to do just that. This advice was given to a lady who has been hypo for many years but has asked for help in understanding her condition.

Somebody is shooting this advice down stating it is wrong and you can not fool the results like that as the lab will take the dose of levo into consideration. So which is right?

Would like to show some support but am questioning which is right

14 Replies

  • The most up-to-date advice is:-

    Leave about 24 hours between levo and blood test. Test should be as early as possible and fast (you can drink water). Take levo after test.

    This guide allows the TSH to remain higher than it would if taken levo just before test and doctors only go by the TSH and ignore T4 or T3 (if they should ever test T3).

    Taking levo just before your blood test skews the results and most probably the doctor will adjust our doses to try to keep our results 'within range'.

  • If the GP is one who only ever checks TSH, it probably wouldn't make much difference.

    It can make a considerable difference if FT4 is being tested though. Taking Levothyroxine just before a test will raise the amount of free T4 in the blood, and might make it appear over-range when it isn't. And as many doctors react to over-range blood tests by reducing a person's dose of Levo, even if the person feel great on that dose, it's to be avoided.

    It comes down to a fundamental lack of understanding amongst many GPs about how the thyroid-pituitary feedback loop works.

  • Its something I repeat ad infinitum

    NEVER NEVER NEVER take thyroid meds in the 24 hours before a blood test

  • Mindlesshaze, it's not about fooling the test, its about not measuring the Levothyroxine peaking in your bloodstream for up to six hours after taking a dose because it will show high FT4 because of the peak which often means GPs and endos will reduce dose. Taking Levothyroxine after the blood draw means normal circulating T4 is measured.


    I am not a medical professional and this information is not intended to be a substitute for medical guidance from your own doctor. Please check with your personal physician before applying any of these suggestions.

  • Hi so if I'm taking my levo at night before bed do the same rules apply or is the 10-11 hrs gap enough not to be a problem?

  • Given that T4 is the storage hormone, it doesn't really matter if you take it the morning before the test, instead of the night before. It doesn't matter that it will be taken about 12 hours after the last dose - some people take their whole week's Worth on one day! The important thing is to leave 24 hours between the last dose and the test. :)

  • Ok thanks, I'm doing a private blood test this week so I will just carry on as normal and do test to see where is sit in the real world. 2 weeks from now I go to first endo app so will do the 24hr approach, could be interesting to see how much difference this is or not.

  • The real world?

  • i've been told to not take meds prior to blood.... but then i think, wouldn't docs want to see how bloodwork is WITH our meds in it? that seems to be more accurate to me.

  • Well, that might be a good idea if doctors knew what they were looking at! But they Don't, and are likely to just say oh, you're over-replaced! And reduce your dose.

    They just Don't have the knowledge to make the distinction.

  • MindlessHaze ,

    I do not understand how "the lab will take the dose of levo into consideration".

    Given that:

    🔴 The lab will only sometimes know you are taking levothyroxine at all.

    🔴 The lab will never know when you last took your levothyroxine.

    🔴 The lab will rarely, if ever, know what dose of levothyroxine you are taking.

    🔴 The lab will not know everything else about the person such as things that might suggest central hypothyroidism, dietary factors, other medicines, etc.

    So how could the lab take it into account?

    You could aways invite the lady to join up here? :-)

  • Glad I wasn't wrong. I am thinking she must be some health professional in NHS, enough said.

    Best thing is she has posted a link which totally goes against everything she has been saying. Think I will post and show my support now

  • I read and commented on the facebook post and also mentioned that her link backed up not taking your levo beforehand.

  • Take any meds AFTER your bloods tests, remember that blood tests only measure whats in your blood not whether its working!

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