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Thyroid UK
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the best time to measure FT3 levels?

OK, so I have a question about so called normal lab results.

The reason I am asking is that I have always read and been told (by doctors and lab technicians alike) that you should go to the lab at least 24 hours after taking thyroid medication. I was told that when I was on thyroxine only, and my doctor (who works with NDT, being a Hertoghe doctor) told me to always wait 24 hours after taking NDT before having labs.

When I was on 5 grains of Armour (1x 300 mg pill a day), I cannot really say I felt bad (I used to chew the pill up before swallowing it). I may not have been feeling great, but I was most definitely fully functional. Anyway, my labs two years ago, 24 hours after taking 300 mg of Armour:

FT4 1.1 (ref 0.7-1.5)

FT3 2.9 (ref 2.2-4.2)

TSH <0.01 (ref 0.5-4.5)

My (Hertoghe) doctor concluded I was under medicated (she only goes by FT3 which, in itself, is rather unusual, I understand...), and told me to increase the dosage. I did, without much result (this doctor uses Armour herself, and does not like Erfa). I should also add that I have diagnosed with adrenal fatigue for which I take 4 mg of Medrol daily.

Anyway, with a prescription for Armour, I was able to get a pharmacy to order me Erfa instead. On 5.5 grains of Erfa (more or less the equivalent of 5 grains of Armour), this is what my labs looked like two months ago:

FT4 1.1 (ref 0.7-1.5) so same as on AT

FT3 4.5 (ref 2.2-4.2) so slightly out-of-range 24 hours after taking meds

TSH <0.01

But, and this is what I have been wondering: if my FT3 levels were elevated on Erfa, even 24 h after taking meds, does this not mean that my FT3 levels were about twice as high the day before; that is, right after taking it? And, if my FT3 levels looked lowish on Armour, could that not mean that they were about 50% higher the day before, right after taking it, and then had dropped?

Don't get me wrong. I am really grateful I have a doctor who listens, who is hypoT herself and takes NDT, and who does not care at all about the TSH (which has never, ever correlated with my symptoms, not even when I was on T4 meds only). But the question I have been asking myself lately is this: what FT3 levels should you aim for if you go to the lab 24 -36 hours after taking NDT? Surely not FT3 levels at top range or above...as that must mean they were much higher the day before...?

I am on Erfa right now, feel that 5.5 grains are too much for me, will decrease it further as I still have hypo symptoms (just talking a brisk walk makes me sweat like I was jogging, plus I have a hard time falling asleep at night, being both tired and wired at the same time), but I have been asking myself more and more which levels to aim for when you have labs? Should I have gone to the lab right after taking Armour Thyroid, instead of waiting 24 hours, to get a true picture of what my FT3 levels look like on Armour?

I feel my FT4 levels are not really an issue, as they have been more or less the same since I went on thyroxine only drugs 15 years ago. My TSH has been suppressed ever since I went from 150 to 175 mcg of thyroxine a decade ago, and I am still very much alive, although my family doctor is freaking out about it every time I see her:-). What varies, and varies considerably depending to which drug I take (Armour or Erfa), are my FT3 levels.

I'd appreciate any input on this!

5 Replies

thecat, leaving 24 hours between dose and blood draw means the normal circulating hormone is measured rather than the peak levels following recent ingestion of a dose. Most T3 will be out of the serum after 6 hours. I was told I could extrapolate FT3 by +20% to estimate normal FT3 levels if I left 24 hours between last dose and blood draw. I've not seen research to confirm this but, if true, your normal FT3 on 3 grains Armour would have been 3.48 and on 5.5 grains Erfa 5.4.

It sounds as though you are overmedicated now. Not everyone feels good with FT3 high-normal or over range. Reducing by 3/4 grain will probably drop FT3 into range. You could speed up wash out of the higher dose by skipping Erfa for a couple of days. You should go on symptoms and how you feel, rather than target lab values.


Thank you, Clutter!!! I have been trying to determine if I should go down the Erfa or the Armour Avenue, and, it seems Erfa is best for me after all...right?! (I just want to make sure I interpreted your post correctly)

You are absolutely right, I should go by symptoms and not "what everyone else thinks". One problem I have noticed lately is terrible itching, indicating that I am overmedicated (just like you say...).

Thanks a lot!


thecat, I can't know whether Armour or Erfa is better for you, but you can't compare the FT3 results on 3 grains Armour with those on 5.5 grains Erfa.


Sorry, I must not have expressed myself very clearly in my previous post, because my lab results were on 5 grains of Armour, vs 5.5 grains of Erfa, respectively...hope this makes it clearer and, once again, my apologies!

I have basically been trying to find out which (FT3) levels you should basically go for when on NDT, especially when on NDT. I know some patients who are on 7.5 or 8 grains of Armour, and doing well on it, and I understand that better now...maybe they need that, especially if they go to the lab 24 hours after latest dose (like every Belgian patient is told to do...).



I wonder one thing in this context: if I raised the Armour dosage, is that likely to raise my FT3 levels as well? Or should I interpret my suboptimal FT3 levels on as much as 5 grains of Armour (measured 24 h after taking the meds) to mean that the cellulose in Armour inhibits T3 absorption, regardless of dosage?

After all, it is rather unusual to have such low FT3 levels (not even mid-range) on as much as 5 grains of NDT, isn't it?


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