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Thyroid UK
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Blood tests

Hi. After reading posts, I've noticed that it's advisable to fast and to not take medication until after the test. Surely this will only provide the doctor with an incorrect result because meds are supposed be taken at the same time every day. That means TSH levels are going to fluctuate and dosing would be incorrect. My Gp and the nurse both advise that I take my meds as usual and the amount of thyroxin I'm taking at the time of the test will be taken into consideration. As a nurse myself, who often discusses testing and results with my patients, I also wouldn't support this pre testing advice. I would never ask a diabetic patient to withdraw insulin before a blood glucose check, same principle applies with thyroxin.

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In all the years I have been on this forum, and both I and others I know have been having blood tests, I have only seen a handful of reports of endos trying to take account of dosing prior to blood draw.

1 They almost never even ask how long before the draw people took their medicine. Certainly, the two people I know best have never been asked.

2 FT4 levels after dosing are not a simple straight line - the typical reported form is a rise to a maximum for about two hours followed by a fall for several hours after. But the actual curve will vary depending on the individual, the make and form of levothyroxine (e.g. liquid, whole tablet, crushed and dispersed tablet) , the effects of food, water taken with the medicine, etc. The height and the shape of the curve both vary.

3 They almost never take into account the other well-known effect of time of day on TSH.

4 I have never been advised by a qualified medic of any sort to even have my blood draw at the same time - nor indeed at any particular time.

In the published medical literature, there is an occasional mention of "trough" levels. However, most papers report thyroid levels without any regard whatsoever to time after dose or time of day. This is a criticism of the the whole edifice of thyroid "science".

In my view, this trough level is a more rational measurement than trying to guess the effect of taking thyroid hormone in the hours before a blood draw.

In an ideal world, perhaps we would see a patient having 24 micro-tests over a 24-hour period with time of dosing clearly identified. You and I both will admit that will never happen - unless someone comes up with something similar to a heart rate monitor.

When you handle a thyroid patient, what do you do to account for time of day and time after dosing? From what you have written, I see two obvious possibilities - you apply your fudge factor, or you ask someone else to apply their fudge factor. You obviously don't ignore the issue.


My extremely senior endocrinologist always advises no thyroxine in 24 hours before testing and no T3 in 12 hours before.

In part I suspect because they know there is too much reliance on blood test results and tendency to reduce dose based on TSH and high FT4 alone.

Obviously if on T3 it peaks soon after taking dose and would give false high

No one ever asks if your taking biotin either on NHS testing

any supplements containing biotin, remember to stop these 3-5 days before any blood tests, as biotin can falsely affect test results



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Perhaps this changes from trust to trust but this is definitely not the case for me.


Levo is a storage hormone, it perfectly possible to take a week's dose all in one go - this actually happens if patient is suspected of being non complient and not taking regularly.

If you tested them soon after taking a weeks dose I should think total T4 result would be interesting!

Insulin is obviously totally different and must be taken regardless of blood test

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Total T4 would be irrelevant - they'd not test it most of the time! :-)

I don't recall seeing an hour by hour TSH done after a week's dose being taken. Wonder what the impact on TSH would be?

I'd also point out that fasting blood glucose is a common enough test - for reasons that to some extent parallel the thyroid hormone issues. That is, blood glucose after food is measuring a different thing to a fasting test.

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Yes I'm well of that. I was simply making a comparison.

Fasting and skipping doses before testing isn't necessary and shouldn't really be encouraged


Your not skipping dose you are rescheduling it (as advised by endocrinologists) to just after blood test rather than just before, if you normally take on waking.

Or slightly longer delay if you normally take at bedtime. You just take dose straight after blood draw, and take that evening's dose as per normal

Bedtime dose also recommended by recent research and endocrinologists


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I agree with you - it is just rescheduling.

I also agree that bed-time dosing seems to be preferred by quite a number of us - not me, I never took it in the morning so don't have that to compare!


I am not sure that I expressed my opinion strongly enough. If the medical establishment had demonstrated their ability to factor in the effects of dosing, of time of day, of all other relevant factors, then we would not be making the suggestion of deferring doses, etc.

(It might be that eventually the science shows that factoring in can be achieved with sufficient knowledge, or that it is impossible to achieve. I'd put my money on the latter.)

They have repeatedly demonstrated their utter inability to take this all into account. We have had patients here who were on the roller coaster of dosing - increased one month, reduced the next - simply because of the relative times of blood draws and tablet taking. And the lottery of appointment times.


It really doesn't matter with levo if meds are taken at the same time every day - in fact you can take a whole week's dose in one go (and GPs do this for "non-compliant" patients) - it's not an active hormone. If you take levo in the morning before a blood test you will be measuring the levo you just took, not the day-to-day blood level. It also causes TSH to nose dive and most GPs think TSH is important even after initial diagnosis and will lower the dose if your TSH is low even if your free T4 is nowhere near the top of the range. If you took it 5 mins before th blood test, I doubt that it would affect the results



Here is previous post by helvella showing the variation in TSH through 24 hours.


If medics treated us on symptoms, not just TSH blood tests, timing would not be so critical.

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