Advice on T3 timing before Test : Hello there. I... - Thyroid UK

Thyroid UK

119,282 members138,533 posts

Advice on T3 timing before Test

Andyb1205 profile image

Hello there. I’m aware of the advice on this forum that T4 should be taken 24 hours prior to testing and for T3 instead the 12 hour rule applies.

I am taking a 40mcg Liothyronine capsule right now. Along with 50mcg Synthroid.

When I did my last test I was taking 25mcg Cytomel so I split it into 12.5 x2 that day and took 12.5mcg 12 hours before the test. Firstly, was the T3 result accurate? Secondly, will testing 12hours after 40mcg T3 be accurate or falsely high?


10 Replies

Andy there lies the connundrum. T3 testing to guide you for medication when your on T3 only is actually quite difficult. Thisis because T3 fluctuates naturally in thebody. When added synthetically as you are it has a much shorter life span than levothyroxine so again depending when you test it too can fluctuate. So the problem is you can easily get widely differing results depending on when the test takes place. Roughly speaking I believe it halfs after 5hrs. If you havent already it would be worth reading Paul Robinsons book Living with T3 only. He also has a facebook group where others on t3 share their experiences.

He said in his book that he and his GP never found the blood test T3 levels helpful when that was used to guide the level of T3 dose. When they did this it resulted in a raising & dropping of liothyronine & he was far less well.

Ive only ever used it as a part of treatment so levothyroxine plus liothyronine and for the last ten years ndt so I dont feel I can comment further.

Am sure a person on T3 only will be along shortly.

Andyb1205 profile image
Andyb1205 in reply to waveylines

Sorry I completely forgot to add that I’m also taking 50mcg Synthroid!

waveylines profile image
waveylines in reply to Andyb1205

Ah different ball game then! 😋 What I do is leave a 24hr period clear as you would for levothyroxine. Test first thing in the morning.

And its my fault as you do say in your original post! My apologies.

Andyb1205 profile image
Andyb1205 in reply to waveylines

No problem! I take my thyroid meds in the evening so I could skip one evening, take next morning and test 24hours.

What I’m worried is if it’ll be falsely low or falsely high. I think the general idea is to multiple the Free T3 result by 1.2 if taken 24hours before. But would like to convince the doctor to try the opposite of what we did before, raise T4 and decrease T3.

Was previously on 75T4/25T3. Free T4 had come back as the same number that the range starts at. Free T3 was 56% in range but I did take 12.5mcg T3 rather than the 25mcg T3 12 hours before the test.

To further confuse this discussion, my functional medicine doctor wants me taking meds (T3 & NDT) 8-9 hours before labs. She doesn't pay much attention to T4 results.

waveylines profile image
waveylines in reply to G2G2

Hmmm interesting. Well my Endocrinologist says dont take my ndt fir 24hrs before testing!! He does test both ft4 & ft3. The tsh is of course supressed.

I suppose if you are on a split medication theres nothing to stop you not taking the levo 24hrs before and do your last T3 dose whatever number of hours before testing.....

Andyb1205 profile image
Andyb1205 in reply to waveylines

I think the reason it’s suggested to take T4 24 hours before testing is to ensure the highest possible TSH. As my TSH was already suppressed on 75T4/25T3, I just might stick with my normal routine of taking them in the evening. Except take it a few hours earlier than bed to ensure 12hours before the test.

G2G2 profile image
G2G2 in reply to waveylines

I don't take Levo. I use NDT & T3. She's the first doctor who told me a time lapse. In the past, I did 14-16 hours. Once, I had to go the ER. When I gave my med list, they insisted on a thyroid panel. To no avail, I explained I'd only taken thyroid a few hours before. Like anyone in an ER would understand. My GP at the time freaked out, cut my meds. He also didn't understand the misleading high T3.

Andyb1205, you asked my question this morning🙂. I posted yesterday (and before) regarding my ft3 levels - I take levo & lio at night about 4am. Since adding the lio I have been taking my lio the evening before the morning blood tests i.e. I’d take my normal dose at night then accelerate the same dose about 8pm for testing in the morning. My ft3 has been over range each time but I have no symptoms that I’d consider to be from being overdosed. I’m not sure if my ft3 reading is what I should be using to inform my dosing as it may be artificially high since it’s possibly not far off being a measure of a split dose of 40mcg as opposed to a single dose of 20mcg. I cannot find any reliable information on the ft3 levels at various intervals after a dose but hopefully responses to your post may be helpful.

IMO, it doesn’t give a false negative - it gives a snapshot in time and that snapshot may be at the lowest point in the day. I got my last test 17 hours after my second, much smaller, dose as it reflects how I take it. I take different doses morning and afternoon and, as I try to take the second one before 3pm, there is a period where t3 will be lower. However, I’m factoring that in when assessing results.

You may also like...