Thyroid Blood Tests: Understanding when to have them taken.

[Foofoo]

Hi - I appreciate this question has been asked many times over, so please forgive me for asking it again:

By all accounts, you should leave 24hrs (give or take a few hours) between your last dose of thyroid meds. and your blood test. At present my endo. has me on 2.5 grains of NDT, split three times through the day (1 grain at 8.00am, 1 grain at 12.00pm and half a grain at 4.30pm).

So, when I go for my next blood test do I take my meds. as usual and get the blood test done at 4.30pm the following day or do I take it all at once at, say 9.00am, and get the test at the same time the following morning? And do I need to fast - 24hrs seems an awful long time to go without any food! (my surgery informs me they can now do non-fasting blood tests for thyroid panels so this might not be such an issue).

When on 2 grains I simply took my second dose at 1.00pm and got my bloods drawn at about 9.00am the following morning, fasting for 12 hours beforehand.

Have I been getting my bloods drawn to early to give an accurate picture?

Thanks so much in advance. Jools

[humanbean]

By all accounts, you should leave 24hrs (give or take a few hours) between your last dose of thyroid meds. and your blood test.

The above is true for people taking Levo. I would always suggest being precise in timing of any thyroid blood tests though - give or take a few hours isn't a great idea.

At present my endo. has me on 2.5 grains of NDT, split three times through the day (1 grain at 8.00am, 1 grain at 12.00pm and half a grain at 4.30pm).

If you have your testing done on a Wednesday (for example), I would suggest taking your NDT on the Tuesday as normal for you. Arrange blood testing for as early as possible on the Wednesday. Don't take NDT on the Wednesday morning until after your blood has been taken.

Fasting - fast for about 8 - 10 hours before the blood tests i.e. overnight and delay breakfast until after the blood has been taken. Water can be drunk freely throughout. Dehydration should be avoided.

Try to stick to the same conditions from now on to maximise the comparability of different sets of tests.

[Foofoo in reply to humanbean]

Thanks human bean, much appreciated.

I did wonder if it might be an idea to take the 2.5 grains split in half, at 8.00am and then 1.00pm before the test as this might give a truer result compared to when I was on 2 grains. Do you think this is wise, or simply keep to what you have kindly suggested above?

Thanks again.

[humanbean in reply to Foofoo]

I think the best idea is to try and test with the minimum of disruption to your normal routine. So don't change your meds the day before, just stick to your usual routine.

Testing done by doctors will often just be a TSH level and this will be the thing they use to decide if you need a change in your dose. (TSH is a pituitary hormone, not a thyroid hormone.) Getting a Free T4 test might happen if an endo orders the test. Getting a Free T3 test is rare.

If you changed your normal routine it would have almost zero effect on your TSH - TSH doesn't change that quickly, but it might just make you feel worse than usual. The thing which has the biggest effect on TSH over a short time period is time of day. A TSH test done at 8am will show a noticeably higher result than a test done at 3pm.

There are some people who say that TSH is lowered by eating which is why we suggest fasting overnight and avoiding breakfast until after the blood draw.

If you take your NDT just before a test that will increase your Free T4 and Free T3 tests, sometimes quite substantially, and those raised results may show up for several hours after taking a dose. So this is the logic behind avoiding meds immediately before a test.

A lot of doctors will want to lower meds if TSH is low or suppressed and this is something most hypothyroid people want to avoid. Unfortunately T3 (or the T3 in NDT) will lower TSH whether we like it or not. If they (doctors) consider the TSH to be too low they may check the Free T4 and Free T3 to see if they are over range (if patients are lucky but they rarely are). So this is why we suggest skipping a dose before a test.

[Foofoo in reply to humanbean]

Thanks human bean, I much appreciate your advice!

[greygoose]

"my surgery informs me they can now do non-fasting blood tests for thyroid panels so this might not be such an issue"

Ha ha! What a laugh! Are there no depths they won't sink to, to avoid correctly treating hypos? I'd love to know how they work that one out, given that doctors have never admitted that you need to fast, anyway!

The point is, TSH drops after eating, that's why it should be a fasting test. I don't see there's anything they can do to stop the TSH dropping. Rather, it's to their advantage that it drops because they can then say 'oh, look, your TSH is too low, we'll have to lower your dose!'

Having said that, the TSH is totally irrelevant, anyway, when you're on NDT. I hope they know that.

[Foofoo]

Thanks GG. Actually, I always fast regardless of what I'm told, so that's what I'll continue to do. I probably haven't painted a very detailed picture but, to his credit, the endo. always insists on FT4 and FT3, along with the obligatory TSH tests. He has made it clear that my TSH will be surpressed due to the direct T3 in NDT and not to make too much of it. It would seem I am also very lucky in that he happily discussed not only T4 monotherapy but also synthetic T3 and NDT. I was also very fortunate that my GP recognised her limited experience in this field and agreed to me seeing this endo. when I continued to complain of symptoms, rather than simply dismissing my concerns (this on synthetic T4 which didn't agree with me). Compared to many others on this forum I think I should count myself very lucky. 😊