Taking NDT before a blood test: Hi, I'm taking... - Thyroid UK

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Taking NDT before a blood test



I'm taking NDT and have a blood test tomorrow morning.

However I might have made a little mistake. I took my NDT as normal this morning before thinking to myself ah how am I meant to take NDT before a blood test.

I did some research and the consensus seems to be to take it 8 - 12 hours beforehand.

So, given that I've just taken my normal dose 24 hours before the test, do people think I should take another half of my daily dose this evening about 12 hours before my blood test?

Hope that makes sense. Basically I'm wondering how to get accurate results having taken my normal NDT dose 24 hours beforehand.

Thanks. Must plan in advance better next time!


15 Replies

I don't think that taking more NDT is the right thing to do. Are you obliged to have your blood test tomorrow? Can you not put it off til another day?

If you have to have it tomorrow, then bear in mind that you will have a false low FT3 - it won't affect anything else. You can mentally add on 20 % of the result to get a total, but probably not going to make an awful lot of difference. It'll give you a rough idea. :)

davidlee72 in reply to greygoose


Ok thanks for your reply.

I can't change the blood test time so I'll just go ahead with it and make a note that the FT3 test result is likely to be a little lower than is actually the case.



greygoose in reply to davidlee72

You're welcome. :)

I take ndt too. I dont take it the day before my morning test. I take it straight after my blood test so its only a 24hr break.


I think the consensus seems to be that you should take the NDT dose 8 - 12 hours before your blood test, and that if you take it 24 hours before your blood test that your FT3 result will be a little inaccurate, i.e. that the blood test result will state an FT3 level lower than it actually is, approx. 20% lower.

Maybe but the nhs only do my TSH & ft4 these days. They used to do my ft3 but been stable on the same full replacement dose for over 10yrs now.

Paul Robinson in his book T3 only states that he and his GP found the Ft3 blood test to be Inconsistent as it fluctuates a lot (which of course it does naturally through the day) so if they treated him by the ft3 result then they were constantly changing his t3 meds and he didnt feel well. He did much better when treated by signs and symptoms. I appreciate in his case its different as the TSH would be sky high and ft4 zero or neglible. However I feel the ft4 alongside the TSH is more reliable unless of course your checking to see how well you are converting.

Its a bit of conundrum but Ive found using a combination of approaches, signs and symptoms including checking temperature/pulse first thing in the morning before rising alongside blood tests to be the most helpful. The Ft3 can only ever be a guide and no more than that due its constant changing of levels in the circadium rhythm.

This is only my opnion and there are far more clever people than me on here who can no doubt advice you better and more precisely.

shawsAdministrator in reply to waveylines

The most important thing the medical profession forget to ask is 'how do you feel'. We can then tell them we either feel well or awful and they are supposed to be the knowledgeable ones to be able to erradicate the disabling symptoms by ensuring the the FT4 and FT3 are in the upper ranges and TSH 1 or below.

waveylines in reply to shaws

Quite agree Shaws!!

I think in general doctors are doing this across the board with blood test tbh. I notice the very same iissue occurs with B12 deficiency!! Lol.

davidlee72 in reply to shaws


Thanks for your message.

You mentioned that we should aim to have the FT3 in the upper range but how can we assess whether it's in the upper range unless there's a consensus on when to take the final T3 dosage before the blood test?

Based on the replies here it sounds like the FT3 test result varies wildly during any 24 hour period.

waveylines, Just to clarify a point, Paul Robinson wouldn't have a sky high TSH with mono-T3 therapy, it would more likely, be very low or suppressed, as the ingestion of exogenous T3 would have that effect.

Oh yes of course Masie Gray your right. My apologies and thanks for correcting. Thats the beauty of forums like this.....having others to make sure what is written is correct. Thanks 🙂🙂

No problem, we all have the duh moments, it goes with the territory 😊

davidlee72 It won't be 'inaccurate' as such, because it will be accurately measuring your FT3 level at that point, given those circumstances; and if that's what you wanted to know, it would be perfect ie how you test depends on what you want to know, or what you want to prove, or as is often in our case, what you want it to persuade the GP to do (increase our dose) or not do (reduce our dose) as a result of the test results. So fasting may help the TSH to be a little higher than it otherwise would be; and leaving 8-12 hrs for T3/NDT instead of 24, may cause the result to be higher, but to a degree it's gameplaying - after all, if it was so finely accurate, then surely we'd need to to test at 8 hrs, or 12 hrs, not anywhere between 8 and 12 hrs. As I take my T3 and T4 together, I stop both together and that gives me consistently the same conditions with each test, and if I do the same but the result is different, then I know to investigate further, irrespective of what that result actually is. Unless we first took a series of hourly tests throughout a diurnal cycle, we can't know with any accuracy, how as an individual, our body exactly reacts to our thyroid meds over time, and how that plays out in changes in levels - we know that different people metabolise Levo differently, and also differently depending on their severity of hypothyroidism.

I've had tests sprung on me at hospital appointments, five hours after my dose, with FT3 over-range: 7.1 pmol/L [3.5 - 6.5].

I take levothyroxine plus Thiroyd, and having my usual morning blood draw 24 hours after the levothyroxine and 12 hours after the Thiroyd, my FT3 was only 4.7 pmol/L.


So many people who're hypo are not aware that 'timing' after taking levothyroxine or any other thyroid hormone replacement, may cause the professionals to reduce the dose, only because the TSH is very low. If they ever test the Frees (rarely) and if on the high side due to not leaving sufficient space between swallowing and testing they'll reduce the dose. It's seems like we walk along a tightrope to try to get our results 'just right' so unnecessary adjustments are not made.

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