When should I take my blood test?

[Hattie87]

Hi, I started taking T3 a month ago (in addition to levothyroxine) and want to take a blood test to see how it's going. I didn't take my T3 this morning (as I felt I was having a little too much) and was going to have my blood test tomorrow. Would this effect the results as it would have been 48 hours since I had any T3? Or wouldn't it make a difference as T3 leaves the body quickly.

Or should I take it tomorrow and go on Friday instead?

Thanks in advance!

[jimh111]

You should have the blood taken half way between doses so that you get an idea of your average levels. After 48 hours is no use as on average three quarters of the T3 will have gone.

[Hattie87 in reply to jimh111]

So ideally in the afternoon? How would that work as I take Levothyroxine too, which I take in the morning. I've heard it's best to get tested in the morning for this one before eating.

[jimh111 in reply to Hattie87]

As long as you leave about three or four hours the levothyroxine won't matter since due to its long half life the timing has a very small effect. Morning testing is suggested in order to get a higher TSH figure. It's more important to get an accurate fT3 figure than fiddle the TSH a little. You don't need to fast, there's a belief that it makes a difference to TSH but I wouldn't go hungry that long. I always have my breakfast, have the blood taken around midday and then go for coffee and a cake.

[Hattie87 in reply to jimh111]

Thank you, would it be worth taking the T3 about 4am as I'm not free then. I haven't taken it this morning, so if I took the usal does at 4am and then went for the test tomorrow morning, do you think that would give an accurate reading?

[jimh111 in reply to Hattie87]

Have you already scheduled the blood test for tomorrow? If not it would be better to leave it for a week or two so that you can be on a stable regime again. It also depends on how much you are taking, if it's only five or ten mcg then taking your L-T3 overnight would probably be OK.

I think it is better to split T3 medicines into two doses, at breakfast and bedtime. If you can only take one dose then before bedtime would be better. I have found the bedtime dose to be important for good quality deep sleep and improving brain function and energy the following day. A single dose at bedtime makes it easier to take it on an empty stomach and helps with timing the blood draw.

[Hattie87 in reply to jimh111]

Thank you very much for your help!