Blood Test

[AppleOrchard]

I’m having a blood test next Tuesday to check my levels of T3 T4 and TSH.

I take 150mcg at night. How long before the blood test should I not take the dose? Last time I took it right up to the test because I forgot and my levels were super high. I’ve had to come down from 175mcg.

Thank you very much.

[Lalatoot]

24 hours

[AppleOrchard in reply to Lalatoot]

Thank you!

[SeasideSusie Remembering]

Last dose of Levo should be 24 hours before blood draw. Adjust timing of dose for a couple of days before when normally taking it at night.

If you take it the night before at your normal time you will have a false high FT4. If you delay your dose until after the blood draw you'll have a false low FT4.

[AppleOrchard in reply to SeasideSusie]

That’s very helpful. I think that’s why I had really high levels of T4. It was the highest it’s been for a long time. Thank you!

[SlowDragon Administrator]

So on Sunday night delay Levothyroxine dose until Monday morning

On Monday delay evening dose until immediately after blood test Tuesday morning

Take Tuesday evening dose as per normal

If taking any supplements that contain biotin (e.g. Vitamin B complex) stop this a week before ALL blood tests as biotin can falsely affect test results

If testing iron levels, stop iron supplements a week before testing

[AppleOrchard in reply to SlowDragon]

Thank you! That’s a very helpful guide. I’ll make sure I do that. It’s difficult to make sure I get it right.

[jimh111]

Levothyroxine gives a spike in fT4 and tT4 for the first four hours after ingestion after that it settles down. So the answer is leave at least four hours. In practice this has negligble effect because the long half life of levothryoxine means we have about 10 days worth of levo in our blood at any time, so one dose has a small effect. As you take your levo at night there will be no problem, there is more than enough time for you blood T4 levels to settle down.

[AppleOrchard in reply to jimh111]

I have always missed the dose before but didn’t last time and my T4 is higher than it’s ever been. I’m not sure why that would be. I’ve been steady for a few years now.

Thank you.

[jimh111 in reply to AppleOrchard]

fT4 seems to jump up on higher doses of levothyroxine, perhaps because less gets converted to T3 or for some other reason. If you leave an extra day add about ten percent to your fT4 result to get a more realistic result.

[SlowDragon Administrator in reply to jimh111]

You may find this is true, but many Hashimoto's patients find they need to leave 24 hours before blood test. This is also recommended by some thyroid specialist endocrinologists