Thyroid UK
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When to stop T3 before blood tests?

Hi everyone

I have a blood test tomorrow morning at 9.40am. Here are the times I normally take my medication:

7am - 100mcg Levo, 20mcg T3

Noon - 20mcg Levo

4pm - 20mcg Levo

I know not to take my medication tomorrow morning before the test and I am guessing I shouldn't take any more T3 today either although I wanted to check as I know it doesn't stay in the system as long as Levo. I will do whatever I need to but I know I am going to feel pretty rubbish without any medication this afternoon as I know when I am due a dose as I start wanting to fall asleep and I get heart palpitations.

Any advice would be appreciated.

Thank you :-)

6 Replies

You could double your noon dose or miss your afternoon dose. This is an excerpt:

Right after taking a T3 containing medication, the TSH level begins to drop and stays suppressed for 5 hours. The TSH level then begins to increase again 5 hours after the dose, and continues to rise until 13 hours after the last dose, after which point it stays stable.

Free T3 levels are also affected, increasing after the dose is given and hitting a peak at 4 hours after dose.


Oh that's great. Thank you so much Shaws.


You have to be careful that your meds aren't adjusted according to your blood test results, i.e. the blood tests were for the use of levothyroxine and might not 'fit' with T3 only. You may be told your T3 is 'too high' etc and that will only refer to the range the labs do which is for levo. Therefore, those of us on T3 only will have a very low T4 and high T3 (they may not test T3 anyway) and usually very low TSH.


If I took my last dose of t3 t4 meds at 4pm and did my blood test at 930 am the next day and my blood test came back with a

tsh of.29 , and bottom bottom of normal results in both ft3 and ft4....

.does that mean I am undermediated.....


Yes, as these should be in the upper third of the range. I would increase your dose.

1 like

thank ; hair is coming out and dr told me to stay on same dose for 8 more weeks and test again......holding to the tsh diagnosis rather than ft3 ft4.....and also ignoring hair loss....I have to find a dr that knows how to treat with nature throid......thank you so much for your reply bec that I what I thought as well from research......and wondered if I just maybe took mymeds at wrongtime b4 test hence dr thinks they were taken that morning bec that is what she demands and she still told me to stay on meds....


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