THYROID BLOOD TEST

[Wayne39]

Can someone please clear up something for me. Is it best to take my last dose of Thyroxine 24hrs before a blood test or is it ok to take my Thyroxine as usual then have blood test?

[jimh111]

Due to its long half life (seven days) taking levothyroxine before the blood test does not have much effect provided it is not taken within a few hours. If your blood test is in the afternoon I would take your morning dose (assuming you take it in the morning). If your blood test is in the morning I would take your levothyroxine tablet after the blood has been taken. There's no need to leave 24 hours which would give results that were a little low but try to leave at least four hours between taking the tablet and having the blood taken.

[shaws Administrator]

We recommended, if on levothyroxine, to allow a 24 hours gap between last dose and test and take it afterwards. It could mean the difference between the doctor adjusting dose down instead of increasing.

If you have levo before blood test TSH will be suppressed and immediately your dose of levo will be reduced. Doctors think a very low or suppressed TSH will cause us to have hert problems. This is an excerpt from another similar post:

RedApple

RedAppleAdministrator

8 years ago

Blood levels of thyroxine are at their peak approximately two hours after swallowing your tablets. They then start to gradually fall over the day.

If like most people, you take your thyroxine first thing in the morning, then your blood level will be at it's lowest just before you swallow your tablets.

It makes sense to me to get your test done when you are at your lowest level. You want to know what your worst scenario is, not your best.

So if you can get your test appointment for the morning, then don't take your medication until after the blood has been drawn. (I usually take my tablets with me and swallow them as soon as I walk away from the phlebotomist!). But if you can't get a morning test appointment, then try to get a late afternoon appointment and take our medication in the morning as usual.

If you are still getting your dose titrated and thus having regular blood tests, consistency of test time is also important. Try to always follow the same testing routine so that you have a reasonably accurate comparison with previous tests.

healthunlocked.com/thyroidu...

[SeasideSusie Remembering]

Wayne39

It depends if the FT4 result is important.

If you take your Levo before your blood test, your FT4 result will reflect the dose just taken so will show a false high rather than the amount normally circulating in the blood. Leaving 24 hours between last dose of Levo and blood draw will reflect the normal amount circulating.

[Wayne39 in reply to SeasideSusie]

I’m having my bloods done through GP. So I will only be having TSH tested!

[SeasideSusie Remembering in reply to Wayne39]

Wayne

Then it will have little effect on TSH because that doesn't measure how much hormone you have circulating. If you're looking for an increase in dose of Levo or to avoid a reduction, then you want the highest possible TSH and that is achieved by blood draw as early as possible in the morning, no later than 9am, and delay breakfast until after the blood draw, plus water only to drink until after the blood draw.

To be able to compare all tests accurately, it's best to always make sure that blood draw is under the same conditions every time. So if previous tests have been early morning, fasting and leaving off Levo for 24 hours, then ensure that future tests are as well.

[Wayne39 in reply to SeasideSusie]

So are you saying because I’m only testing TSH that I can take my Thyroxine as normal in the morning before my blood test?

[Hidden in reply to Wayne39]

I would not take it. Often the lab technician draws extra tubes of blood in case the doctor adds some last minute tests. Then the tech may double check with the doctor to see if he wants to add any tests. This is not uncommon and gives the doctor a chance to rethink his original order. You may not know what they are. Skip your thyroid meds as Shaws described and you will be covered if he orders last minute thyroid tests.

[Hidden]

I would take Shaws' advice and leave at least 24 hours between your last dose and your test. Don't mention this to the doctor or lab tech. Unfortunately as many doctors tend to undermedicate we patients have had to learn to make things easy to get a therapeutic dose ordered for ourselves. It's unfortunate we have to do this to get the dose we need but the system itself is not very honest, in my opinion. Just advice I follow for myself. And my dosages work fine. In a perfect world absorption times would always be accurate but we all absorb at different rates and we can have different half-life times. These can't be measured so we allow for the maximum margin of error. irina