T3 & T4 blood test timing

[Faith442024]

Hi guys 👋

I’ll soon need to take my first blood test after starting a combination of T4 and T3.

I’m a bit confused about the best time to schedule the test.

I know that T4 should be tested 24 hours after taking it, while T3 apparently should be tested 10–12 hours after the last dose.

For context, I take T3 three times a day—at 8 AM, 12 PM, and 3 PM—and I take T4 at 8 Am.

Hence T3 is short acting, can l still test at 8 AM, as is common for T4 testing, or do I need to adjust the timing due to my T3 schedule if yes how on earth can l do that?

Thank you in advance for any replies 💐

[Buddy195 Administrator]

Forum advice is to get the earliest possible time for a thyroid blood test & have only water prior to the test. Also ensure that last Levothyroxine dose is 24 hours before the blood draw. For T3 split this the day before and take last dose 8-12 hours before the test. This would mean delaying your last dose of Liothyronine until the evening and taking Levothyroxine after the blood draw.

[Faith442024 in reply to Buddy195]

Thank you Buddy195 👍

[SlowDragon Administrator]

Testing 8am

Day before test change timing of T3 to approx

8 AM, 2pm and 8pm

Delay T4 dose until AFTER test

[Faith442024 in reply to SlowDragon]

Thank you SlowDragon l will do exactly that 👍

[greygoose]

Just one point here: T3 is not short-acting. That is a misconception that endos are broadcasting far and wide! It's not true.

The half-life of T3 in the blood - where it is inactive! - is about 24 hours. That means that if you took 10 mcg T3 this morning (just an example) by tomorrow morning there will be about 5 mcg left - minus what got into the cells. And then you take anothing 10 mcg so there will always be some in your blood.

What gets into the cells stays there for about three days, and is of course constantly topped up.

The point of leaving a gap of 8 to 12 hours between your last dose and the blood draw is so that you get and idea of what your usual circulating level in the blood is (we cannot test how much gets into the cells) so that you know if it's too high or too low, because symptoms alone can be confusing. Even so, blood tests are never 100% accurate and ranges are only a rough guide, but it's the best we can do. So, if you take blood test results into consideration with symptoms they will guide you as to whether you might need an increase or a decrease in dose. Or if it's pretty good where it is and should be left alone.

[Faith442024 in reply to greygoose]

Thank you, Greygoose, for your datailed response and for correcting me regarding T3 😊

[greygoose in reply to Faith442024]

you're welcome.