Thyroid UK
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How Long Should wait after taking t3 for test?

Hi All,

on 23rd of december i had my free t3 tested, it was FREE T3 6.3 [3.1-6.8]

I took t3 for 2 weeks to see if it raised my body temp

I got it tested again 3 days after my last pill and it came in 4.9 [3.1-6.8]

Is it possible I didn't wait long enough for my own thyroid function to kick in again? The past few times in past when I had t3 tested, it was always right at top of range

note: I also started taking iodine ( 50mg day )

also, does this active b12 look low. I'm a 29 year old male

Active B12 57.6 [ 25.1 - 165.0 ] N

4 Replies

Sorry you have had no replies - hopefully someone with relevant knowledge will pick this up from Latest Activity...




T3 has a half life of about 8 hours. While not taking your normal dose on the morning of testing is correct, not taking it for 3 days means you have utilised any 'stored' T3 and would explain why your recent T3 result has dropped.

I'm not sure whether supplementing iron when on thyroid meds is recommended or not. I was under the impression that thyroid hormone replacement should make it unnecessary to supplement with iodine.

I think your B12 looks low. You shouldn't supplement without having your folate level checked first. I think supplementing B12 may mask low folate but it might be the other way around.


I took 25mcg of T3, I'm waiting on my reverse T3 result and cortisol result. Then I will try again


Hi, yes my free t3 level is fine.. however I have a TSH of 6.33 but my ft4 is also at top end of range.. and with my low body temps, I thought taking a bit of t3 might help. I now realise 25mcg is not enough


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