Thyroid UK
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Stop t3 for blood work?

Hi everyone,

I have to do my blood work and I’m not sure if I need to skip my t3 the night/ day before? I’m on 10 mcg now but only I just started the second dose at 5 pm. So it’s 5 2x per day. Except I stopped the second dose for now due to palpitations and being on an antibiotic.

Thanks!

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When taking T3 you leave 12 hours after last dose when having blood draw for test. Any longer and you get a false low FT3. T3 peaks in the blood 2-6 hours after taking it so this would be reflected in the result if you take your T3 before the blood draw.

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I have often wondered about this question myself and have had a variety of answers which included longer times post T3 dose e.g. 18 hours.

I cannot see how it's possible to test other than privately and at home, if you take your last T3 dose, as many would do, at say, 5.30pm. You would need to be taking your last dose at 9pm at least to enable you to have the test done at the Dr for 9a.m.

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I just alter the timing of my dose the previous day.

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Oh right I see - you just take that last dose much later then.....

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I take Levo plus T3. Normally I take them before I get up, I don't split my T3.

When I test then the day before I split my T3 and take part of my normal dose when I wake up and the rest around 9pm-ish. I don't notice any difference whether I split my dose of T3 or not. I usually do my home tests around 8.30am.

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I am taking virtually no T4 now, so not sure how I would manage with a change of regime for a dose on testing day as the T3 of course doesn't stay in the system long.

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I normally take last dose T3 about 11pm with Levothyroxine, so just bring it forward to 9pm.

Remembering also NOT to take my Levo at 11pm.

Then get blood drawn at 9am (local private hospital), straight after take Levo dose, delayed from night before and normal waking dose T3.

Pop blood test in post on way home and wait an hour for breakfast.

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That all sounds perfect if you can do it like that.

I am only doing NHS tests at present, and not had one of them for a while. I have no car so getting to the surgery is a pain if it's early, though not impossible. As I said to Susie, not sure how I would cope with changing my T3 doses though as my energy drops quite quickly if I don't have a dose at the right time, since I have very little T4 with it. Currently only 12.5 a day. I will give it a go next time though!

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Thanks I also take t4 levo. I take them at 2-4 am usually so I’ll take them earlier and skip the 2nd t3 prolly at 5 pm. For levo I just did my normal dose and time at 2-4 am.

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If you skip the 5pm dose of T3, then if your last T3 dose was 2-4am the day before the test, and you're having an early morning blood draw the next day, say 9am, then that is far too long, it would be over 24 hours. You could take your 5pm T3 at around 9pm the night before the blood draw, then take your next dose after the blood draw.

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Well I’ve only been taking it once per day because I’m on Zithromax and I was getting more palpitations with an extra 5 mcg. I’d only done the extra dose for 3-4 days so I’ll just go back to once a day till I get the blood work. Are you saying it will affect the test if I just take it at 2-4 am? T4 doesn’t seem to affect the test, but who knows for sure.

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When measuring our thyroid hormone (the Free Ts) it's recommended that we should leave 24 hours for Levo and 12 hours for T3. If you leave it any longer you will get false lows, if you take them too close to blood draw you will get false highs. We want to test what is normally circulating in our blood so those timings are recommended. If necessary, adjust timing of doses to get as close as possible to these suggestions.

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Interesting. I don’t think I can go 24 hours without levo because I get a migraine if I do.

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