Thyroid UK
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Blood test for those on t3 only

Firstly, thank you very much for this very informative site. I have learnt much from your experience. I have one question. Next week I am going for the blood test to see if/how my self dosed t3 only therapy is working. The test will be done around 8 am. Should I take my usual morning dose of 6 mcg at 6.30 am before the test? When I was on t4 only before, my endo always asked me to take the dose first thing in the morning before the test to see the affects of it in my blood test. Now, if I do not take the morning dose of t3 before the test I know the result will probably be slightly hypo, as I usually feel the wearing out after 4-5 hours.

I am on 15 mcg t3 only, split in 2 or 3 doses daily and feeling better than on t4. On the other side, If I take the dose before the test, I will have the idea how it affects my blood. What do you think?? Thanks and have a nice day.

3 Replies

Being on T3 only can suppress the TSH in some people and of course the FT4 reading will be low too if your thyroid is not producing. I have always read here on the forum - not to take the medication before a test - but afterwards.......


It is usually recommended a gap of around 24 hours before a blood test. Obviously that's for one daily dose. So if you take a morning dose, you take it after the blood test. If you take an evening dose, you miss this and take after test.

My personal opinion is that it's immaterial of what's in your blood. It's how you feel that is the most important fact of how the medication is working. T3 will be high in your blood for several hours after ingesting. It then goes into your receptor cells which should be saturated and it's work begins then, sending out 'waves' for between 1 and 3 days. So I don't understand your Endo's thinking.

Your dose of 15mcg is very small, I don't see the point of splitting it. It's roughly equal to about 60mcg of levo. So, for the day before the blood test, I would take your two doses before noon and take your doses as usual after your blood test. An excerpt:-

The Thyroid Manager online textbook mentions that

Serum T4 concentrations peak 2 to 4 hours after an oral dose and remain above normal for approximately 6 hours in patients receiving daily replacement therapy.

For this reason, thyroid expert Richard Shames, MD has the following recommendation:

I absolutely recommend that patients have any morning blood tests evaluating the thyroid before taking any thyroid medication. I have always told my patients to do it this way.

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Thank you Shaws. My dose of t3 is small because my t4 dose was also small (50 mcg) but the more my endo raised it the more tired and foggy I felt, from the very beginning. With or without t4 I did not feel right. I made an rt3 test which was high so I went on a 2 month trial of t3 to see how it is working. So far so good. I was afraid to go on higher doses, well, at least, so far.

Next week I will make a new blood test to see if my ft3 is higher. On 50 mcg of t4 my ft3 was 3.8 (ref 3.1 - 6.8) and ft4 was as high as 19 ( ref. 12 - 22). My ratio ft3 / rt3 was 8,55.

So, no t3 dose before the blood test. Thank you guys.


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