Does FT3 increase when T4 does?

So basically, I am currently on 125mg Levothyroxine. My results before christmas were something like TSH 0.8 and T4 16.6. However I went for my next blood test and also asked for FT3 whist I was having them done. The reults cam back at:

T4 20.2 (range 12-22)

FT3 5.9 (range ~3-6.6)

TSH 0.23 (range 0.3 - 5)

So my question is: when T4 increases does this make the T3, meaning that if my levothroxine is decreased my T3 will? If so is it worth enquiring about adding T3 to my medication?


11 Replies

Hello Emma

Yes, T4 is a storage hormone and this is converted into T3 which is the active hormone which does the actual stuff your thyroid hormones are supposed to do so changing T4 will effect the level of T3 - I'm sure someone else will explain it better than me by tomorrow lol.

Your labs look pretty optimal to be honest so it looks like the right dose but everyone is different so may not be optimal for you. You seem to be converting well so not sure you need to add T3? Do you have any symptoms you are concerned about? Some people do have an issue with T4 regardless of their tests so depends on how you feel?

I would say that because the TSH is a little under range, you may find they will want to reduce your medication even if you feel better. If you feel good at this level, try to refuse.

Have you had B12, Vit D, Ferritin and Folate levels looked at?

Ah I just thought of a good analogy if my attempt of an explanation failed and confused you further. If our body is a steam engine and T4 is water and T3 is steam, the water (T4) is converted to steam (t3) and this runs the engine. The more water you have, the more steam you have and the faster the engine runs.

I should have left it to the pros me thinks lol ;-)

That makes it easy to understand .... never thought of it that way. Lol


I have never had those checked. Everytime the levothyroxine is changed, I feel better for about a week but then nothing. I also have issues in that I am having a tough time losing weight.

You results are very good. If you aren't feeling good, you need to look at vitamins, minerals, and general nutrition and physical fitness.


Welcome to our forum.

Your results are good, indicating good conversion so you do not need to add T3.

125mcg T4 is not a large dose and shouldn't be reduced but I assume your GP is mistakenly concerned over your low TSH.

The goal of Levothyroxine is to restore a euthyroid status and many require a lower TSH in order to function well.

If you feel unwell, ask your GP to test Vit B12, folate, ferritin and Vit D. Deficiencies are common in hypothyroidism and can affect how thyroid hormones synthesis.

How long have you been medicating and have you had thyroid antibodies checked ?


I have been treated for over a year now. But everytime I still don't feel right. I have had the antibodies checked and I was positive. They did this before I was treated. 😊


So are you on a gluten free diet / no dairy or sugar. Also have you been told you have hashimotos?

Radd is correct we all need optimal vitamin levels- if your gp won't test them look at blue horizon to have them tested privately. They make a massive difference.

It's also worth reading Dr MYHills web page on adrenals just to have more information. Many people do have both conditions.

Good luck


I have read about the gluten free diet but haven't had the willpower to give up bagels haha.

I was told by the doctor that it's autoimmune. My family has quite a few of them.

I will look into further tests. Back at the doctors next week. I will also give that a read.


Most thyroid problems are autoimmune and this is hashimotos, this is where gluten free can help as it can cause inflammation and trigger autoimmunity.

You can get nice gluten free bagels from certain places lol. Well if you continue to feel unwell and all else has been ruled out and you're at a loss, you could just give it a trial for 2-3 months. If it makes no difference, just go back to eating it. I can honestly say I feel so much better, I don't miss it at all :-)

I agree your results are what I would test is your vitamin d3 level , your b12 level which run low and make you feel want to be in the upper level of normal to feel great...I take a b complex, a d3 5000 with k2 in it daily.....I know when mine was low I was so tired and achey......exhausted......very common for them to be If you do a blood test....those are the first two that jump out....that make you feel still hypo if low....ferritin iron is another.....

you can have low stomach acid which makes you don't absorb vitamins from food, you could have cortisol issues from stress but they usually effect thyroid test so I think you probably don't have those issues....can you be more specific in symptoms you are may want to google low vitamin d or low vitamin b12 and see if you see yourself in the list....

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