How much NDT can skew blood test results?. TSH ... - Thyroid UK

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How much NDT can skew blood test results?. TSH 2.9, T4 - 14, T3- 7.5 ( above the range)...

I had blood tests on 26th of Feb. Unfortunately blood was taken 3 hours after I took 1/2 grain of ERFA.

Tests were done unexpectedly, in a hospital and frankly I did not have any energy to argue with blissfully ignorant doctor that taking NDT before Thyroid Function Test can skew the results.

Any way the results are as follow:

TSH 2.9 ref. range ( 0.3-5)

T4 14 ref. range ( 9-25)

T3 7.5 - ref. range (3.5-6.5) so at that point above the range.

At the time of above blood tests I was just on 3/4 gr of ERFA. I took my morning dose of 1/2 grain then approx. 3 hours later my blood was taken ( as mentioned above)

I feel dreadful and frankly beyond 'fed up'. I am on 1 grain of ERFA now, I have lowered the dose from 1 and a 1/4 of a grain after advice that I might have been increasing too fast.

My temperature do not jump up and down as much now ( still below normal), I have chest pains, fatigue, permanently sore throat and an incredible brain fog or rather a mental block, my memory is gone totally.

According to some hypothyroid sufferers particular kind of what's called a 'brain fog' can be caused not only by low T3 but also too much T3...

Does anybody know ( perhaps Rod?) how much ( approximately) the reading was influenced by NDT.

I am waiting for next tests results, and I pray that my surgery and lab perform tests that I have requested.

Thanks for help.

E xx

7 Replies
helvella profile image


I would love to be able to answer your question usefully. This is my best guess - but it could be way out.

I would expect you to have been only slightly past the peak of both T3 and T4 blood levels. But we all know that individuals vary quite a lot. Typically blood levels peak somewhere around one to two hours after swallowing. Inevitably eating, drinking, chewing (or not), etc. all have their effects.

TSH should not be that far out because of the Erfa - but is likely to be significantly lower than it would have been at, say, 08:00.

Please folk, don't hold back in answering, or contradicting me. I would far rather someone did that than leave Edysia with only my answer.

All the best


in reply to helvella

Thank You so much Rod. I do appreciate your help. Perhaps if I was tested slightly after the peak than they are not skewed that badly. I was just curious if anybody came across similar situation.


PinkNinja profile image
PinkNinja in reply to

I agree with Rod in that I don't think TSH would be influenced too much (and it is still higher than you will probably need it to be anyway) but the T3 result probably was influenced significantly. What I have read in the past correlates with what you have said and it makes logical sense.

I wonder if there is an element of the T3 not getting into the cells or not being used by the cells at the moment (hence a high T3 reading), perhaps because they have been deprived for so long (I have no idea if I read this too, or just imagined it - brain foggy day). If this is the case, then a more gradual increase sounds like a good plan. This would also result in a more skewed blood T3 result.

Carolyn x

your blood test result is 'skewed' ie: does not show what is really going on as you took the NDT 3 hours prior to the test. In future just 'decline' the blood test, you don't have to have a reason to decline it nor have to explain anything just say I do not want it/do not consent/do not fancy having it done today. 'end of'

I never have bloods any earlier than 24 hours after my last dose of T3 (NDT when I used to be on NDT) if any doctor says 'lets do bloods' and I took my medication in the previous 23 hours I say 'no, I do not with to have blood drawn today' and if he asks why I say 'I just don't, am too tired, will have it done next week'.

Your bloods do not tell you what is going on at this stage.

in reply to nobodysdriving

Thank you NBD. I would decline if I had energy and the right frame of mind to decline. The problems is I was caught in a very vulnerable place and I was too weak to think clearly.

I know that they are skewed hence my question of how much they can be influenced? I will keep searching medical literature, I hope to find some frame of reference.

nobodysdriving profile image
nobodysdriving in reply to

well, I doubt you will find anything too 'specific and clear cut' however I would think that almost any blood test done any earlier than 12 hours post T3 (and NDT contains T3) will show too high levels of fT3 in the blood.

in reply to nobodysdriving

Thank you for your warm response Nobodysdriving.

Best wishes

E xx

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