Free T3 results when taking T3 only and when to take last dose before test

I took a private blood test last week (Blue Horizon Thyroid +10) mainly to get a FT3 result as the NHS isn't keen to do this any more, at least in my area (even though GP requested it). I wasn't too sure when to take the last dose before the test but was told to leave 24hrs in between dose and test, which I did. The result was 3.76pmol/L (3.1- 6.8). Although this is in the normal range it's not great? However, I have since read that, in the following article, it is more accurate to take the test six hours after a dose (I take four doses of 5mg T3-only per day = 20mg and have been on T3-only for seven years):

Any comments on my result and when to dose before a test gratefully received. Also, I have been playing around with the total dose for five months (lowering dose) trying to stop palpitations and had only been back on 20mg for about a week before the test. Would this have had an effect? So many questions!

14 Replies

  • Glamrocks4girls,

    You need to be on a dose for 6 weeks before testing to judge whether the dose is right. If you've been on a lower dose it's likely your FT3 is still building.

    If you extrapolate your 3.76 result by +20% it gives 4.51 which is probably close to your normal circulating FT3 level. 12 hours gap between last dose and blood draw is sufficient when on T3 only and +10% extrapolation will give you the estimated normal level. I would only do a blood test 6 hours after last dose if it was for my own information. I wouldn't care for my endo to decide that FT3 is a bit high if the T3 is still peaking and reduce my dose on that basis.


    I am not a medical professional and this information is not intended to be a substitute for medical guidance from your own doctor. Please check with your personal physician before applying any of these suggestions.

  • Thanks Clutter. It would seem I should have held off that test for a few weeks! Are you saying that 4.51 is more likely to be where I am when on 20mg for at least six weeks? Sorry, but I am a little unclear about the 20% extrapolation.

    I am still confused about optimal timing for the test. Dr P says 24hrs, that article suggested 6hrs. I guess 12 hrs is a good compromise but where does the 10% extrapolation come from?

    With all the factors involved I don't see how you can ever get an accurate reading.

  • Glamrocks4girls,

    No, I'm saying 4.51 is likely to be your FT3 level if you hadn't left 24 hours between last dose and blood draw.

    T3 peaks in the blood stream for up to 6 hours so most people are temporarily overmedicated after taking a dose. You are spreading the peaks and troughs by taking 4 x small doses.

    Every reading is accurate as a thyroid test is just a snapshot in time. Six hours after last dose is probably the most accurate in terms of judging whether your dose is right if you are prepared to risk a result showing slight over medication if the T3 is still be peaking and that your doctor may reduce dose if you appear to be over medicated. 12 hours gives a safe margin that the T3 has stopped peaking.

    If 24 hours after last dose FT3 was 3.76 adding 20% to the result gives a realistic estimate of 4.51 as your normal circulating level. If you leave 12 hours add 10% to the FT3 result.

  • Hi , What does your t4 level look like if you only use t3?

  • Bettan,

    Completely below range.

  • Hi , I take 20MCG t3 twice a day: 5am and 3pm. My last blood test was: *ft4 <2 pmol/L ; TSH 0,33 mu/L ; ft3 4.3 pmol/L (no heterophilic antibody interference was found when checked)

    * " Abnormal, pt has F/U appt ! Serum free T4 level

  • Does the same apply when you take combined Levo/t3? My latest test 12 hours after t3 & 18 hours after Levo

    TSH 0.08 (.27-4.2)

    T4 16.45 (12-22)

    T3 was 5.12 (3.1-6.8) so my normal circulating level would be 5.63. Does that leave room for improvement on t3?

  • Pinklady756,

    Yes, it's the same for T4+T3. Your results look good to me but there is room to increase FT3 if you are still symptomatic as long as you keep it <6.8.

  • Thank you Clutter. If I add another 6.25mcg t3 should I reduce Levo?

  • Pinklady756,

    FT4 is only mid-range so I don't think there is any need to reduce Levothyroxine dose.

  • Great thank you 😀

  • Thank you so much for explaining Clutter. I shall take it all on board for any FT3 test in the future. So do you think 4.51 is a reasonable result? All the other thyroid results were out of range: TSH 6.6 IU/L (0.27- 4.2) T4 Total 28.5nmol/L (64.5 - 142.0) Free T4 4.24 pmol/L (12 -22), not surprising after a 24-hr gap between medication?

  • Glamrocks4girls,

    You are undermedicated to have TSH 6.6. On T3 only most people will have suppressed TSH <0.1 with FT3 in the top third of range >5.5. FT3 is currently reasonably good but that is probably due to high TSH flogging the thyroid to produce and convert T3. FT4 is below range as expected when you take T3 only.

  • Thanks Clutter. In previous recent tests I have been around 5 for TSH. That would have been after taking the last dose (10mg, as I used to take the 20mg in two doses) at 11.30 pm the night before a test. I assumed the even higher TSH this time was from the longer delay (24hrs) between doses. Neither my GP nor Dr P seemed very bothered by the high TSH readings (around 5).

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