taking T3 = always lower FT4 levels?

It has been stated here repeatedly that when you take drugs containing T3, be it NDT or synthetic T3, your FT4 levels will drop, so you cannot compare them to what they looked like when on T4 only drugs. I know that is true as that has happened to me. What I don't understand, though, is WHY the addition of T3 will cause FT4 levels to drop...to prevent excessive rT3? (I'm more curious than anything, it does not really matter as long as I feel well).

16 Replies

  • Anna69,

    I think the addition of Liothyronine (T3) encourages better conversion and use of the T4 and that's why FT4 drops.

  • Interesting theory, thanks!

  • It's because the body only hangs on to what it wants, and excretes the rest, as far as I know. However, when certain people - including endos - insist on trying to raise it, the excess excess can be converted to rT3 in order not to waste all that good T4 - the body doesn't like waste.

  • Equally interesting and makes perfect sense! I interpret that to mean that there is no reason to try to raise FT4 levels if FT3 levels are good...?

  • Not as far as I know, no.

  • Sorry, have to disagree with greygoose. If the body only used what it needs, then no one would be hyper. Clutter's answer is correct. T3 stimulates T4 to T3 conversion. That's why people on T4-only tend to have low T3, and adding T3 seems to lower T4--more of the T4 was then converted to T3.

    The body uses both T4 and T3, so if you're short of EITHER one, you'll be hypo at some level. There are some good charts on this page that illustrate how your particular medication affects your results: tiredthyroid.com/rt3-3.html

  • Thanks, that's also very interesting! I have been wondering about this a lot lately as my FT3 levels are good on NDT (I use Thyroid-S).

    I have been following Clutter's advice to expect your FT3 levels to be ca 20% higher on the previous day compared to what shows up on your lab sheet, provided you take your meds once a day as I do (in the morning, so I have not taken any meds for 24 h when going to the lab). So, when I had labs in August '16, my FT3 levels were 2.9 (ref 1.7-3.7). If they were 20% higher on the previous day, that would put them around 3.48 which has to be considered good (in upper part of range but not out of range). At this time, I felt good.

    However, my FT4 levels were at bottom of range; 0.9 (ref 0.8-1.5). I cannot seem to get them up; if I raise Thyroid-S, my FT3 levels end up 4.6 (ref 1.7-3.7) 24 h after taking NDT. But my FT4 levels were good: 1.2 (ref 0.8-1.5). For the sake of convenience, and because I thought there was room for it, I added a whole grain which, in retrospect, may have been a mistake as the pills can be cut in halves and even quarters...

    TSH completely suppressed (0.01) but that has been the case for at least ten years and does not seem to affect me at all. Since I have Hashimoto's, that may even be a good thing.

    I have been wondering lately if I should try to get my FT4 levels up by adding some pure T4 to the mix, but then there is always the risk of increased or even excessive rT3 which I'd like to avoid...?

    I know pigs produce the same hormones as humans but in different ratios but, in patients not converting T4 to T3 (which I don't seem to do because, before going on NDT, my FT4 levels were at top or even slightly out of range, yet, my FT3 levels remained at bottom of range even on as much as 200 mcg of T4 daily), maybe it's a good thing that we are getting more T3 and less T4 than our thyroid glands would normally be giving us...or should you always take additional T4 if your FT4 levels remain low on NDT, but you cannot seem to add more NDT without your FT3 levels ending up out of range...?

    Some US doctors working with alternative treatment methods seem to advocate the addition of T3 only drugs rather than T4 drugs.

    Any thoughts or comments or personal experiences with either method would be greatly appreciated!

  • I personally take NDT + T4 and feel better on the combo than taking either alone. You could try adding 25 mcg to your NDT, and see how you feel. For me, brain function and sleep improve with the additional T4.

  • Thank you so much; yes, that is what I'll do! I have been struggling to find a balance that I simply doubt can be found on NDT or T4 alone...maybe for the simple reason that pigs produce less T4 and more T3 than humans...?

  • Yep, that's it!

  • If your thyroid is still working to some extent then taking T3 will lower TSH and the T4 from the thyroid (and thyroidal T3) will fall.

    If your thyroid is kaput then T4 will only come from tablets and so will reflect your reduced T4 intake. So if you take NDT instead of levothyroxine then your T3 (total or free) will be higher and T4 lower.

    If your NDT or L-T3 dose is higher than your equivalent L-T4 dose type 3 deiodinase will increase and more T4 will be converted to rT3 and T3 to T2.

    I've never seen anything to suggest that taking T3 would increase T4 to T3 conversion, it doesn't make sense. The opposite will happen, type 3 deiodinase will increase. If it didn't patients with hyperthyroidism would have their condition greatly aggravated by even more T3.

  • I honestly don't expect my own thyroid gland to be working at all anymore; I was diagnosed with Hashimoto's in 2000 and have been on thyroid hormone replacement since (first, T4 only, then, synthetic T4 + T3 and, finally, NDT). At the time of diagnosis, my TSH was 19 (ref 0.5-4.0), and FT4 0.5 (ref 0.8-1.5). FT3 never measured as deemed unimportant. Anti-TPO >6000 (ref <8).I had mononucleosis in 1995 and, looking back, I realise all my metabolic problems started back then...I slowly but surely started putting on weight, I started losing a lot of hair, my nails turned brittle, my skin dry and, five years later, I was diagnosed with Hashimoto's. Those symptoms did not appear overnight, though, but over the course of several years. But, looking back, I can clearly see mononucleosis being a dividing line. I read much later that mononucleosis is a common cause of autoimmune hypothyroidism.

  • I had mononucleosis in 1999 and my autoimmune problems showed up in 2007.

  • So jimh111, if my thyroid is still working, I won't have a very low FT4 level when on NDT?

  • If your thyroid is OK and you take a little NDT your TSH will fall and you will produce less T4. If you take sufficient NDT to suppress your TSH all your hormone will come from the NDT, you will generally have lower T4 levels. However as the T4 is coming from the NDT your T4 will be in proportion to how much NDT you take (as will be your T3).

  • Thank you!

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