When To Take Throid Hormone For Blood Test

I was called in recently by my GP practice for a review of my thyroid hormone prescription. My usual GP has retired and I saw a doctor who said it was very unusual to be on T3 and she had not done a test for it before!

I asked her what she wanted me to do about taking my L-thyroxine and Liothyronine before the test as I have had different instructions from different doctors. She said to take my normal dose before the test which will be at 9.45am. I usually take it at about 7.15 - 7.30 am.

I am wondering what the correct procedure is?

9 Replies

  • The maximum free T4 level from a levothyroxine tablets occurs about two hours after taking it.

    The maximum free T3 level from a liothyronine tablet occurs about one and a half hours after taking it.

    Both slowly drop in the hours thereafter.

    It would, in my opinion, be madness to test at almost exactly the time at which both fT3 and fT4 will be at their artifical highest. That measurement would show your absorption of the active ingredients. If I can put it this way, you need to have sufficient so that you are not plummetting into hypothyroid levels before your next dose!

    Usual view here is not to take any thyroid hormone for at least 12 hours, probably better 24 hours, before a blood draw.

  • That's what I thought Helvella. That is what I did before but my T3 levels were always low and my GP said it was because I had not taken any since the evening before. ( I have always taken 5mcg before my evening meal as instructed by a private Functional Medicine doctor I was seeing who was reputed to be an expert in hypothyroid)

  • I think you mean that you took 5 micrograms at dinner time and had blood drawn around 12 or so hours later?

    If so, yes your T3 levels would be lower than in the two or three hours after taking the tablet. But far more representative of your typical levels.

  • Yes, that's correct.

  • Make the appointment as early as possible and fast - you can drink water. (Taking food decreases TSH and might result in GP adjusting dose as most only take notice of the TSH and diagnose accordingly.).

    Taking thyroid hormones before a blood test skews the results and may cause a reduction in dose which may be completely unnecessary.Leave about 24 hours between last dose and blood test and take hormones afterwards..

  • Shaws, I already told the GP that I was one of those who had a normal TSH with a low T3. My TSH has been low of course ever since I have been taking T3. She did not want me to do a fasting test. She called it a baseline test. I may cancel the test and speak to the GP again before doing it. Don't want to waste the NHS funds.

  • I would take your normal dose of L-T3 the night before (assuming you take it more than once daily) as the fT3 figure can go haywire if the blood is taken within a few hours of taking the medication. I would let you doctor know and when you get the fT3 figure allow for an extra 10% - 20% to get an idea of your average level. This is what I do and it avoids the assay being distorted by recently taken L-T3 and also avoids a falsly low figure due to the short half life (~24 hours) of T3.

  • NEVER take any thyroid hormone before blood tests, it will skew the results as blood tests only measure what's in the blood not whether its working.

  • Quite so Glynisrose. That's what I have been told in the past. This new GP seems clueless. I can only do what she says, wait for the result, see what she says about it and take it from there. All I want is for her to carry on prescribing my thyroid hormones.

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