Thyroid UK

I know this is a stupid question but.....if I lower my T3 dosage will my FT4 reading go up??

What I mean is...

I take 100mcgs of levo giving me a reading of. 15.5 (12-22)

I take 20 mcgs of T3 giving me a reading of 5.2 (3.1-6.8)

Is taking T3 suppressing my T4 level at all? And if I stop taking my T3 what will happen to my FT4 level? Will it go up, down or just stay the same?

Somewhere in my head I seem to recall reading that taking T3 lowers the FT4 level but is this only true if you don't supplement with T4?

Please , please, I know it's a ridiculous question but I'm having one of those days!

It's starting to feel like one of those maths questions at school that I dreaded..😩

14 Replies

The active hormone is T3. And in the range you have given you are in the top 1/3 of it which is good.

T4 is a precursor to T3 and as you are taking T3 your FT4 will be lower as your body will not need to convert T4 to T3.

Anyway how do you feel?

The reason I'm asking this is to find out why you want to change your medication.


It's not a stupid question at all. There are no stupid questions if you really want to learn.

It's not that taking T3 suppresses T4, it's that the body only keeps what it needs (within limits), and it doesn't need as much. So, it doesn't hang on to it.

Are you saying that you want to reduce your T3 to bring your FT4 up? Why would you want to do that? It doesn't matter that your FT4 is low in range, because you're taking T3 'ready-made'. :)

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Hi greygoose,

It's just that I don't feel well until my FT3 is about 6.2.

I am a way off that yet. I need to be in the upper quartile to feel symptom free.

At the moment I feel dreadful. In the evenings my eyes start swelling up again and I just want to sleep. It's like everything is running out. To be honest I don't feel well during the day either.

I hate T3 !

I would much prefer to be on all T4 but my endo says that I have to have some T3 in the mix.

I'm not going to come off anything. Just asking that's all.

My endo did say that we could increase my T4 by 25mcgs to lift the FT3 a bit.


That's where most people need their FT3 to be. Nothing unusual in that. It's low T3 that causes symptoms.

Why do you hate T3? I don't understand. And why does your endo say you have to have some T3? If people are good converters, they can live quite happily on T4 only.

Do you know how well you convert? That's a pretty important thing to know. But, you can only tell that when you're on T4 only. Have you even been on T4 only?


Thanks greygoose,

I'm not a good converter. On 100mcgs of levo I have an FT3 below range.

I am secondary hypothyroid.

My endo says that in order to get a good FT3 level for me to function properly I need to put the T3 in the mix.

The alternative is to increase my levothyroxine till it takes my FT4 levels way over range in order to give me a decent FT3 level. But I would imagine that would be an awful lot of thyroxine?

I'm so heartened by you saying that most people seem to need their FT3 levels at about 6.2 or high in range.

When others say just above half way is good I could just weep. I feel so poorly at that level and it makes me despair thinking that I must be odd or strange or at the very worst....untreatable.

Thanks for the support.


So, what was your FT4 on 100 mcg levo? It's by comparing the FT4 level with the FT3 level that you know how well you convert.

But, if you don't convert well, there's absolutely not point in increasing the T4. Not only would it take a lot of levo to get enough T3, but you would end up with a lot of unconverted T4 slopping around in the blood. And, that's not good.

So, if you don't convert well, your endo is right, you do need T3.

Who says that just over mid-range is good? Not someone who is hypo, I bet! It might very well be good for someone who is euthyroid, but you can't compare healthy people with hypos, you just can't. When you are hypo, you need your levels higher. I need mine just over the top of the range.

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Hi greygoose,

I've found some old results from when I was on 100mcgs of Levo ONLY.

FT4. 13.9. ( 8.27-23.1)

FT3. 3.70. ( 4.17- 9.46)

TSH. 0.35. ( 0.46-4.68)

What do you think?


I don't think your T4 dose was high enough to really tell. You didn't have enough T4 to convert.


Really. So i would just have to try more T4 to find out?

Don't worry I'm not going to do anything without the endos input.


Considering how difficult it is for people to get prescribed T3, I think you should just hang on to it, and thank your lucky stars! lol


I know. i was so aware of that when I wrote this. I thought people are going to think I'm stark raving mad!

Like being offered a pot of gold and turning it down for pig iron! 😳



But, of course, you have to take enough of things to get the benefit from them. It's not the fact of taking it that does you good - as so many doctors seem to think! - it's taking enough. And you aren't taking enough.


How do you FEEL? Thats more important than numbers.


Well in short... rubbish! But still smiling..😁


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