How Off Would Blood Levels Be If Tested Without Fasting Beforehand?

[MindfulMoments]

Hiya. I would appreciate thoughts on this. This morning I had to go for an Inflammatory Marker Blood test for something which isn't related to thyroid issues. Unknown to me, my GP had requested a thyroid blood test anyway so I wasn't prepared by fasting from my meds the day before, and I took a multivitamin the afternoon before as well. Will this mean my results will be totally out? It's really annoying because I wanted a thyroid test to see how my new batch of ndt is doing, but now I won't be able to see the exact results and I won't be able to request another test now 😒.

[helvella Administrator Thyroid UK]

The need to fast for thyroid tests has always been questionable.

My view is simple.

Let us consider that you decide not to fast and have several thyroid tests. All consistent in every other way.

Then you have a thyroid test at the same time as another test which needs you to fast. So this time your thyroid tests are in a fasting state.

When you get your thyroid results, you see they are a little different to previous tests. Was that due to fasting?

I really don't think it is possible to tell. It might be. It might not be. Your thyroid levels might really have changed.

By always fasting, you ensure an equivalence from one test to the next. You remove the possibility of fasting vs not fasting being significant.

However, any differences from fasting which might exist are small. If differences due to fasting/not fasting were large, this would by now be mentioned far more widely, and you'd hope the testing systems would tell us.

[MindfulMoments in reply to helvella]

Thanks helvella, hopefully, the results will give me somewhat of a proper indication as to where I'm at. Thank you!

[RedApple Administrator]

' I wasn't prepared by fasting from my meds the day before, and I took a multivitamin the afternoon before as well.'

This is a bit confusing. Do you mean fasting from food (which is what fasting usually means), or are you saying you took your thyroid medications before the test?

[MindfulMoments in reply to RedApple]

I didn't fast from my thyroid meds this morning, meaning I took my meds this morning as usual and yesterday afternoon, including a multivitamin yesterday.

[SlowDragon Administrator in reply to MindfulMoments]

If you took your meds before test you will get false high Ft4 and Ft3 on NDT

[MindfulMoments in reply to SlowDragon]

That was what I am afraid of.

[SlowDragon Administrator in reply to MindfulMoments]

On plus side NHS rarely tests more than TSH…..and almost never Ft3

[RedApple Administrator in reply to MindfulMoments]

If your thyroid test was for TSH only, it probably won't be significantly affected. But if FT4 and FT3 are tested, they may show as being too high or at least higher than usual.

[MindfulMoments in reply to RedApple]

Thanks, I always get T4, T3 and TSH done. I will have to see,and explain to my GP the reason why if they are high.

[RedApple Administrator in reply to MindfulMoments]

GPs frequently do not understand or believe that taking thyroid meds before the test affects levels, so be prepared for that!

[MindfulMoments in reply to RedApple]

Yes, thanks, I know all to well about that. It's a real battle talking about thyroid issues with most conventional doctors.

[jimh111]

"so I wasn't prepared by fasting from my meds the day before". Do you mean not eating or not taking your meds? I believe fasting makes no difference but the timing of taking thyroid hormone, specifically T3 will make substantial difference.

As an unscientific guess I think the best approach is to have the blood taken about half way between T3 doses and at least three or four hours after the last dose.

[MindfulMoments]

I didn't fast from my meds, meaning, I took my meds this morning as usual, around 6.50am and the test was at 8.40. I normally refrain from having any meds after my morning dose the day before a morning test. I didn't do that this time because I was unaware the GP had requested it as well as the other blood test which was nothing to do with the thyroid. I'm hoping it won't be too out.

[Sparklingsunshine in reply to MindfulMoments]

Do you get your NDT via the NHS, or privately/ self source? Unless you are beholden to your GP for your prescriptions I shouldnt worry. And if you are explain you weren't expecting a thyroid test and ask for it to be re tested.

[MindfulMoments in reply to Sparklingsunshine]

Thanks, I self medicate so dosing isn't an issue. I just would have liked to see how I'm doing.

[Sparklingsunshine in reply to MindfulMoments]

I thought as much, its annoying when they spring a test on you like that. I'm sort of surprised your GP does NHS tests if you arent taking Levo, unless you haven't told them 😉

[MindfulMoments in reply to Sparklingsunshine]

The one GP does try his best within the NHS constraints. He knows I self medicate and agrees to test my T3 once a year, but says he can't do anymore.

[jimh111 in reply to MindfulMoments]

You will get a false high fT3 result. On the other hand you have been getting false low fT3 results by skipping the previous afternoon doses.

[MindfulMoments in reply to jimh111]

Oh, I thought you had to. Thanks, I won't miss it it out next time.

[DippyDame]

I don't think there is an exact way to test NDT levels

For T4 leave a 24hr gap between dose and testing

For T3 leave a 12 hr gap between dose and testing

For NDT a compromise is required since it contains both T4 and T3....consensus seems to be 8 or 12 hrs gap

A single test is a snapshot of a moment's hormone level, since they can vary throughout the day.

Testing at the same time of day, for every test is preferable ( for comparison) and testing at 9am gives the highest TSH reading of the day and therefore the lowest hormone level....but not a foolproof result.

TSH is a pituitary hormone not a thyroid hormone....it's a signal to the thyroid gland to produce or reduce hormone production. But, that signal doesn't specify which hormone's level needs to be altered ( T4 or T3) which is why we need both FT4 and FT3 labs

So... perhaps the best way to monitor NDT which avoids fluctuating levels is to keep a record of all signs and symptoms.

Your GP probably only tests TSH in any case and science proves that is not a reliable marker.

How you feel is the most important factor!

[MindfulMoments]

Thanks. I normally have my 1st dose of the day, the day before my due test which I book as early as possible in the morning, and do not take my afternoon dose, nor any vitamins that day. It's a pain why the GP didn't tell me when he told me he was going to do the other test. I will just have to see what the results show.

[greygoose in reply to MindfulMoments]

It would be a good idea to always check with the nurse that takes the blood exactly what they are testing. And refuse any tests you don't want. They can't force you to do blood tests you don't want, that would be assault!

[MindfulMoments]

Yes, in hindsight, I should have asked if I could postpone the thyroid test until next week when I could be prepared. I could have kicked myself so to speak not thinking to do that. Oh well, what's done is done. You live and learn 😄.