When to take the last T3 dose before a blood test. - Thyroid UK

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When to take the last T3 dose before a blood test.

saritadelmar
saritadelmar

Just asking for some clarity about the statement I hear often on this forum, to wait 8 to 12 hours before a blood test after taking the last dose of T3.

Question: Should one actually time the last dose of T3 to be 8 to 12 hours before the test? Or if taking T3 in the morning of the day before blood test; will 24 hours after that dose be too long without T3 in the blood, and result in too low FT3 on the test?

I'm hoping I have asked this question clearly, and hope it doesn't seem a stupid question. Thanks in advance.

16 Replies
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SeasideSusie
SeasideSusieAdministrator

Question: Should one actually time the last dose of T3 to be 8 to 12 hours before the test?

Yes

Or if taking T3 in the morning of the day before blood test; will 24 hours after that dose be too long without T3 in the blood, and result in too low FT3 on the test?

Yes.

If you normally take your T3 in one dose, split it into two doses the day before the test, take the second dose 8-12 hours before the test.

Thank you for the clarification. I will take it at night then, which I have been avoiding, because the Endo said it might keep me awake.

I am taking a very small dose of 5mcg, which I have split, because it caused some anxiety when I took it all at one time. I take 75 Levo at 7:30 am, and then half hour later 2.5 mcg T3 and again at 2:00 pm another 2.5 mcg. This has been working for me.

You are saying I should take the last T3 at about 9 pm the night before my test at 9 am in the morning. Seems like such a small dose. Any thoughts about this? Or confirmation I am correct now?

SeasideSusie
SeasideSusieAdministrator in reply to saritadelmar

Take the last dose 8-12 hours before the tests so 9pm the night before is OK for a 9am blood draw.

Thank you! Will do.

Hidden
Hidden in reply to SeasideSusie

According to jimh111 (elsewhere) if you split your daily dose, the blood test should be done between the two doses on that day. Not sure I go with that.

saritadelmar
saritadelmar in reply to Hidden

In order to do that, as jimh111 says, I would have to change the time of my first dose as well as the last. It would also mean that I would not be able to take the blood test in the early morning. I think I will follow SeasideSusie's advice and just take the last dose at 9 pm.

Hidden
Hidden in reply to saritadelmar

Very wise 😉. I spoke with tongue in cheek.

saritadelmar
saritadelmar in reply to Hidden

Thank you for your input!

bessygo
bessygo in reply to SeasideSusie

To clarify for myself, my Free T3 was 2.7 (range 2.3-4.2)

and Total T3 was 99 (range 76-181).

HOWEVER, my last dosage of 2.5 mcg T3, prior to fasting blood labs was just over 24 hours. I didn't know about the 8-12 hour effect of T3. Therefore, is that why my T3 may be a bit "artificially" low since it was more than 24 hours since last dosage? Thanks so much for all your knowledge and feedback. Invaluable!

SeasideSusie
SeasideSusieAdministrator in reply to bessygo

Yes, your FT3 will be lower than the normal circulating amount due to last dose being so long before the test.

Total T3 isn't useful test, it shows the total amount of bound and unbound (free) T3, it's the free t3 that tells us what we need to know.

bessygo
bessygo in reply to SeasideSusie

Thanks. Since I am on such a low dosage, I don't split the dosage.

So is it really necessary to take 1/2 of a 2.5 mcg T3, 8-12 hours prior to a fasting lab to get a more "accurate" reading?

I know these levels fluctuate and it seems almost like splitting hairs. I didn't realize Total T3 wasn't useful.

Should one also get Reverse T3 instead?

SeasideSusie
SeasideSusieAdministrator in reply to bessygo

I haven't taken such a low dose so I don't know how much difference it would make and I have no idea how you could split that, so my suggestion would be to just take the 2.5mcg 12 hours before the test.

Your current FT3 result suggests you could increase your dose, that level would be too low for most people.

At the end of the day it's how you feel that's important.

Total T3 is said to be an outdated test and is not used in the UK, it's free T3 that we test.

Reverse T3 is not worth testing. There are many, many reasons for high reverse T3 that are not connected with thyroid so if you do test and the result comes back high it doesn't tell you why it's high. If it was connected with thyroid you would have a high, probably over range FT4 and low FT3 when on Levo only, and that would tell you that you're not converting T4 to T3 well enough. But you are taking T3 so your results can't reflect your natural conversion.

Take the blood about half-way between doses and you will get a rough average result.

saritadelmar
saritadelmar in reply to jimh111

This is good to remember if and when I change the time of my dose. Thank you.

Interesting you should ask this as I am in a sort of similar position but think differently from the advice you have been given, and, btw, do you mean 25 mcg of T3 rather than 2.5?

My endo has told me to leave 4 hours between taking my last dose of T3 (I take 10 mcg 5 times per day) and having the blood drawn.

If you take Levothyroxine as well, perhaps you can leave longer as suggested, I take only T3, but, having worked in a hospital, I know that the lab is informed of the meds you are taking and the results show this. So, the doctors and labs adjust for your meds when reading the test results, if you see what I mean. So, my TSH is always around 0 but the endo and lab see at as fine because I take T3, a person on T4 would not be seen as ok with a TSH of 0 (although some people are).

Since you take T3 as well as T4 (Levo) and you want your T3 reading to be accurate, I'd think carefully about leaving longer than advised by your own endocrinologist.

My thinking is that if we leave 12 hours, then our T3 result is artificially low and we may then be over-medicated with the accompanying risks.

This is just an opinion, not a disagreement with other viewpoints.

Thank you for your comment. I think it makes sense in that 12 hours does seem long. I wonder why 8 to 12 hours is something that is promoted on this forum. My endo most likely would say it doesn't matter - I've asked about it when only taking Levo. I think I will call though and ask, just to get her opinion, as I am getting closer to the time for a blood test.

I have just started on T3 and starting with a very low dose of 5 mcg, so yes I am splitting it and that makes the dose 2.5 mcg. Maybe I will just take the last dose 8 hours before instead of 12. Thanks again.

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