Blood test - fasting?

I’ve heard that I should fast before thyroid blood tests due to the effect of eating on suppressing TSH. But, if fasting will give me a higher TSH reading, will it also give me a lower T4 (and T3) reading? What about the effect on reverse T3, free T3 and free T4?

[AngleX]

I’ve just saw your post and posted a similar one. Thanks for that. I have my bloods Wednesday morning. X

[waveylines]

Think you will find that the fasting is a requirement in order to give an accurate result. The manufacturers of levothyroxine instruct you to not take your meds on the morning of the test but take them afterwards. The test should be first thing. On the nhs they test TSH, ft4 and very very rarely ft3.... nothing else. You are not doing this everyday but just on the blood test morning. It therefore should not affect you greatly.

[Hidden in reply to waveylines]

Thanks for your reply. It is a private test, not an NHS one. Sorry, I should have mentioned. I am having tested:

TSH

FT4

TT4

FT3

RT3

RT3 ratio

TGAB

TPOAB

Active B12

Folate

Vitamin D

But, the results I am most concerned about are my numbers of FT3, FT4, TT4, RT3 and RT3 ratio. I’m not as bothered about the TSH, though I have never fasted before a TSH test so would be interesting to see if fasting does bring it up even more (but, not at the expense of fasting bringing up my T4 and T3 or screwing with my RT3 result). So, I want to know: would fasting result in my results for TT4, FT4, FT3 being higher than if I had not fasted? And, will fasting alter my results for RT3 in any way - such as making it lower?

Thanks!

[greygoose]

We fast before having a thyroid blood test because TSH drops after eating. And, as doctors usually only look at the TSH, we want it to be as high as possible in order to get a diagnosis or increase in dose. We then continue to fast, even after the diagnosis, so that we can compare results - to do that, they all need to be done under the same conditions.

As far as we know, fasting does not have any effect on other thyroid tests, such as FT3 or rT3 or antibodies. But, it is very doubtful that any research has actually been done on this.

[Hidden in reply to greygoose]

Thank you. It’s just that my previous blood test was non-fasting. My TSH was mostly fine (2.5), but my FT3 and FT4 were scraping the bottom of the normal range. I’m having this blood test mainly to see what state my RT3 is in. It would be annoying if fasting somehow screwed with that... I will have to do more research on the matter.

[greygoose in reply to Hidden]

Well, it really depends who the test is for, and whether you want to be able to compare with other tests. But, honestly, I have to say, you're wasting your money with that rT3 test. It won't tell you anything you can't find out by cheaper means.

Oh, and a TSH of 2.5 is not mostly fine. If you are not yet on thyroid hormone replacement, it shows your thyroid is seriously struggling. If you are on thyroid hormone replacement, it shows that you are under-medicated. A normal TSH is around 1.

[jimh111]

Fasting does not affect TSH or fT4 ncbi.nlm.nih.gov/pmc/articl... . Unfortunately this write up of this study is not clear, you need to look at the comments regarding 'Delta 1' and 'Delta 2' to see the effects of fasting. Timing of the blood take does affect TSH but beware it is your body clock not the clock on the wall that really has effect. It is better to have the blood taken later in the day when the TSH cycle has levelled out if you want an accurate and repeatable results.

The time of the menstrual cycle has far more effect on TSH than time of day frontiersin.org/articles/10... .

We shouldn't pay too much attention to blood tests for diagnosing hypothyroidism. If TSH is very low it is a warning sign and we should take more care. A low TSH may be justified if the patient is still clinically hypothyroid.

rT3 is a pretty useless assay and waste of money. The actual figure doesn't mean anything, if fT4 is high or there is illness or depression rT3 will be high. The 'rT3 ratio' is pure quackery, comparing a total rT3 with free T3 is just silly pseudo science.

[greygoose in reply to jimh111]

Totally agree with you about the rT3, jimh111! (But not the rest. lol) Stupid test that gets people all lathered up about something they don't really understand. I think we have STTM to thank for that!

[Treepie in reply to greygoose]

GG I take it you dismiss the Suzy Cohen article linked a couple of days ago"Measure Reverse T3 and get Thyroid Healthy!"

[greygoose in reply to Treepie]

I haven't read that article, I'm afraid.