Thyroid levels are about the same with or witho... - Thyroid UK

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Thyroid levels are about the same with or without Levothyroxine


Can anyone help me understand why my thyroid results are about the same with or without levothyroxine?

I have had 2 recent thyroid blood tests, the first was while I was taking 100 /125mcg (alternate days) and I took my normal dose on the morning of the test.

March 2017

(Taking 100 or 125mcg of levo on day of test)

Serum TSH <0.05mu/L (0.35-5.50)

Serum free T4 21.3pmol/L (11.0-23.0) SR

I then read it is advised not to take a dose for 24hrs prior to the next test as this can skew the results, so I followed this advice for the second test.

June 2017 (Taking no levo 24hrs prior to test)

Serum TSH <0.05mu/L (0.35-5.50)

Serum free T4 20.7pmol/L (11.0-23.0)

But I am confused as the results are about the same for both tests, is this usual? I thought the results would differ so does this mean I shouldn't be taking levo? is this why I feel so ill all the time?

Sorry for all the questions and appreciate any insight on this! x

10 Replies

You don't need to leave 24 hours before having a thyroid blood test. As long as you leave a few hours the test will be fine, enough time for the tablet to be absorbed and not affect the assay. Beyiond that the longer you leave the less accurate the test gradually becomes.

Levothyroxine has a long half-life, about sevens days. So you will build up many days worth of thyroxine in your system. Each daily dose has a small effect. By leaving 24 hours, instead of say 6 hours, your fT4 will be about seven percent lower, close to the accuracy tolerance of the assay. So you will not really notice a difference between the two blood tests. Just don't have the blood taken within a couple of hours of taking the tablet.

Beefeathers in reply to jimh111

Hi Thank you for taking time to reply although it has left me more confused! I have read quite a few comments / advice to leave a 24hr gap.

jimh111 in reply to Beefeathers

I think they leave a 24 hour gap to get low numbers so that they can persuade doctors to give them an increase. Many doctors put too much reliance on the blood tests.

The important thing is to always leave the same length of time, so that you can compare results. Not 24 hours one time, and six hours the next time. :)

Hi - if you post the figures from those two tests with ranges (numbers in brackets after each), then we may be able to help with why you don't feel better. Also if you have any results for vit d,vit b12, folate and ferritin please include thos. You can add to your original post by hitting the down arrow button at bottom of post and selecting edit.


Thank you Gillian, I have updated with the results added, I have had the following done as well

March 2017

serum B12 433ng/l (180-640)

serum folate >24ugl (3-20)

serum ferritin 57ug/l (20-200)

June 2017

Serum 25-HO Vit D3 level 64.6 nmol/L >50

Thank you :)

Ok - so your results, whilst close, are not exactly the same but those ones don't really matter. The important one is ft3 which is the active form used by the body. Your Tsh and ft4 numbers would suggest that you should be feeling better as the ft4 is almost at the top of the range, but it could be that you don't convert well to ft3 and hence these could be a bit deceptive. You really need an ft3 test.

As to your nutrients, your b12 and folate both look reasonable, though b12 could go a bit higher for optimum. Your ferritin is a bit low, optimum would be considered to be 100-120 with a minimum of 70 for thyroid to work properly. You would need to supplement that with iron & vit c tablets. Your vit d is also too low, optimum is around 100-130 and again that could be causing issues of its own - normally felt as aching muscles and joints - but will also be hindering conversion and use of thyroid hormones. I would suggest 3000iu per day for 3 months then re-test. There are important co-factors to that, vit k2-mk7 and magnesium. For much more detailed advice including references for more info, dosing levels, timing, interactions and sources, look under vitamins &minerals in "topics" box at hrs of screen and find answers by SeasiseSusie - she is our real expert on this.

Good luck


Thank you Gillian, and a huge thankyou to SeaSideSusie who has actually helped me on a couple of other threads re the vits.

The thyroid results had confused though, especially as I had read not to take levo on the morning of the test so presumed the results would be very different.

I am looking into private testing for ft3 as nhs lab wont do it.

Thank you so much for explaining, I am fairly new to the site and finding such a lot of great information here (and support) the information is slowly sinking in! :)

jimh111 in reply to Beefeathers

The thing about taking levo (or liothyronine or NDT) before taking the blood is that about an hour after taking it the free hormone levels jump up as it is absorbed but not had time to bind to transport proteins. So, an hour or two after taking thyroid hormone you get spurious results. With levothyroxine it doesn't make a lot of difference (about 8%) whether you leave three hours or twenty three hours.

So, exactly how long after you took the levo was the blood draw? There is a slight difference, but the size of the difference will depend on the length of time. If you swallow the tablets immediately before the blood draw, for example, there would be no difference, because the T4 would not have had time to get into the blood.

But, I agree with startagaingirl, the reason you don't feel well is probably because you aren't converting that T4 to T3.

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