Surprise Thyroid Test

[Harley4]

Hi,

Just wondering if anyone has seen any information regarding how thyroid blood test results are affected if medication is taken morning of the test?

I’ve just had a totally unrelated referral appointment and when running my bloods they mentioned doing a full thyroid panel as well (including T3!). I didn’t think anything of it as I’ve checked them recently and they’re fine…it wasn’t till I was driving home that I realised, the blood was taken at 2.30pm and I’d taken both T4 and T3 meds this morning so they are going to be off - I’m just curious as to how bad the results are going to be 🙀

Unfortunately, the results will eventually get back to my GP so I’ll expect a call. I’ll point out the anomaly in comparing the results against my other ones on file but could do without it!

[tattybogle]

So The TSH result will be affected by the time of the test 2.30pm , this is when the TSH is lowest every day (between about 1pm to 3pm) it is highest overnight/ very early am an d then gradually falls to mid afternoon then gradually rises again , so 9 am TSH results are naturally a bit higher than 2pm results .

(the difference may be as much as 40% in some people .. in others it may be negligible . also if TSH is very low/ supressed it is unikely to change much over the day)

The time of the last dose T4 / or T3 won't affect the TSH level significantly on the day of the test . TSH is more long term reflection of thyroid hormone levels .

However the fT4 and fT3 results WILL be affected by the time of the last dose.

if you took them both at 7 am ish . then tested at 2.30pm that is 6 hrs gap.

Levo will raise fT4 to a high peak in blood lasting from about 2-6hrs after the dose was taken

T3 will raise the fT3 to a high peak in a similar fashion.

after approx 6 hrs these high peaks will slowly start to fall.. but those timing are a guestimate because the speed of everyone's digestion/ absorption is different.

But basically if you normally leave 24hrs from levo, and 8-12 hrs from T3 before testing, you can expect these fT4 and fT3 results to 'look' higher than they usually do on the same dose.

and if you usually test at 9 am you can expect this TSH to 'look' lower than it usually does for that dose .

which can give an uninformed GP an excuse to say "ah ha , i see you are overmedicated because your FT4/ FT3 are higher than usual and your TSH is lower than usual"

This post includes research references ( 3rd/ 4th reply down) which explain the effect of time of last dose/ time of day and eating breakfast on blood results: healthunlocked.com/thyroidu...

This graph shows the effects on fT4 /fT3 in the hours following last dose T4/T3 combo ,, vs levo alone on a sample of people taking part in research.

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[Harley4 in reply to tattybogle]

Thank you so much, that’s exactly what I was looking for.

I’m not too concerned about my TSH as it’s suppressed anyway, I am just hoping my T3/T4 don’t go flying out of the top of the range. They were mid range when I tested a couple of weeks ago so have a low buffer. Worst case scenario, I’ll argue that they need to re-run the test based on my private results and see what happens.