blood test 22h after last Thiroid med

[Orso111]

Is it ok to do a blood test 22h after last Thiroid med…I have no choice as I will be travelling later. Does it make a huge difference?

[SeasideSusie Remembering]

Your FT4 and FT3 results will be false lows. You can allow for this but there really is no way of working out what your accurate normal circulating hormone levels are, anything will be a guesstimate.

It's normal to split dose the day before and adjust times to get the correct time gap from last dose to blood draw.

What time is your test?

[Orso111 in reply to SeasideSusie]

Thank you. In short, useless so won’t do it. So tell me if I understand correctly. The day before the blood test (D0) I take at 9 am half of my curent medication (75/2) and I can do my blood test at 9 am (D1).

Sorry if it is a rocky question.

Cxx

[SeasideSusie Remembering in reply to Orso111]

The day before your test you take your full normal daily dose but you split your dose into 2 or 3, whichever suits you. Take the last portion of your dose between 9pm and 1am (8-12 hour gap before blood test).

[Susieg1 in reply to SeasideSusie]

So if I took my NDT 24 hours before my blood test, my T3 and T4 numbers are falsely low? Why is that?

Would that make my TSH falsely low too?

[SeasideSusie Remembering in reply to Susieg1]

FT3 is the most important result, and because NDT contains T4 and T3 then we time our dose to give as accurate a measure of FT3 as we can because this has a short half life of about 24 hours. T4 has a much longer half life of about 7 days. So to avoid a false high or false low FT3 result we need to time the dose so we don't measure when it peaks in the bood (2-4 hours after ingestion) or we don't measure when it's at it's lowest (24 hours after ingestion). FT4 may be affected but not as much as FT3.

TSH is not affected by the time you take your thyroid meds. As you're taking NDT and because it contains T3 then TSH is going to be low anyway, that's just what happens with T3.

[greygoose]

What sort of meds do you take?

[Orso111 in reply to greygoose]

TCAPS 75µg caps

[greygoose in reply to Orso111]

That's NDT?

[helvella Administrator Thyroid UK in reply to greygoose]

TCAPS are levothyroxine.

TCAPS – capsule molle Lévothyroxine sodique

Laboratoire Genevrier, Antibes, France

75 microgrammes, capsule molle

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[greygoose in reply to helvella]

Thank you. Never heard of them before.

[SeasideSusie Remembering]

Orso111

As Thiroyd is the name of a brand of NDT I assumed what you had written in your heading meant that you were taking that brand of NDT. It now seems that your thyroid medication is Levothyroxine. In that case it is fine to take it 22 hours before your blood test, it is close enough to the 24 hour time gap advised to not make any difference.

[Orso111]

Thank you. Following your advise I did it 23h after.