Thyroid UK

rT3/FT3 ratio

My ratio 2 months ago was 21 on 75 mcg T4 only.

Since then I have tried going T3 only but I feel better with T4 in my system.

However since my ratio was 21 on such a tiny amount of T4 then on 200 mcg T4 and 18 T3 the ratio must have gone to an unhealthy level under 20. I was feeling more and more hypo on 200 T4.

I may be one of those people who should go T3 only and will always have a reverse T3 problem when on T4 but I cannot tolerate T3 alone.

Does this mean I am bound to be hypothyroid for life?

5 Replies

I'm not sure it works like that. There's nothing linear about reactions to anything thyroid-related. It depends why you were converting to rT3. Everyone has some rT3 in their system, it's a safety valve for the body. And, ratios are for healthy people, they don't necessarily have the same meaning for hypos. It's not having rT3 that makes you hypo, it's low FT3. How low was your FT3? And no level of rT3 is 'unhealthy' in and of itself.

You have problems converting, which means that your FT3 was low. Therefore you really don't want 200 mcg T4, that's silly. However, reduce your T4 down to 50, and increase your dose of T3 until you feel well, and rT3 probably won't be a problem.

I often think it's a mistake to even measure rT3, because people get so hung up on it. You know very well how well you're converting by comparing your FT4 with your FT3, and you can adjust your doses accordingly. It really doesn't matter that much, how much rT3 you have. You see what I mean?

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Thank you, greygoose!

I needed some consolation since today I am feeling very ill and blue.

At that time on 75 T4 my FT3 was 3.1 (2-4.4) and my rT3 was 143 (90-215), both in same units - pg/ml I think. My FT4 was 19.5 (12-24).

Now I discontinued taking 200 T4 for 4 days and now I am on 50 T4 and 18 T3. However I cannot raise T3, I don't tolerate it.

I am waiting for a Thyroid-S shipment to arrive. I may try 1 grain of it, along with 12.5 synthetic T3 which will make 38 T4 and +/- 20 T3. I hope I will tolerate T3 in NDT better than synthetic.

I really feel much better when I add T4 do T3. I really don't think I can go without it. Adding it makes a great difference.

So you are suggesting that as long as I have a healthy, robust FT3 level then I should not be bothered with rT3 level and rT3/FT3 ratio?


As long as you feel well on that dose. But, if you're a bad converter, you do not want huge doses of T4. 200 mcg was just silly.

If you are going onto Thyroid S, do make sure your nutrients are all optimal, or you won't be able to use it well. In fact, low nutrients could be why you can't increase your T3.


I don't feel well at that dose (50 T4 + 18 T3) at all. I feel very hypothyroid. My FT3 is not even mid-range... Whatever I do, I cannot raise my FT3 level.

I am supplementing and my levels are good with the exception of ferritin which is only +/-15. However, my iron is 23 (10-30) and transferrin saturation is fine. I really don't know how to raise ferritin.

I know that one of the reasons I cannot raise T3 is low ferritin, however I cannot take anything for it since my serum iron will go over range. Even if I take little iron, to raise it from 23 to 30, I doubt it that ferritin will reach 70.


Actually I don't think that anyone can convert optimally on T4 alone, even a good converter per se. On T4 alone TSH falls down and slows down conversion as well. FT4 goes high. You cannot trick the Pituitary feedback loop.


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