Thyroid UK
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What happens to T4 if you take T3?

I am currently taking 2 grains Armour with 15mcg T3. I think that the additional T3 means that the T4 in Armour is not being converted to T3 but RT3?

The reason I ask is that I was currently on 2 - 2/14 grains nearly a year ago and think I possibly went hyper. Now my current dose it a lot higher than that so maybe my body is not creating (much) T3 from the T4?

7 Replies

Yes , unused t4 has to be disposed of, so its broken down into reverse t3, this allows the body to save the iodine and rt3 is inactive.

It then breaks down again into t2 and then t1... Iodine is needed to make new hormones so its stored everytime,

Neat eh?

Xx g

1 like

Hi Galathea,

Thanks for the quick response. I don't have a thyroid so was wondering what governs this?


Its the liver and kidneys which do the breaking down and the pituary which governs thyroid hormone.

So if you have too much t4 Or t3 in the blood the tsh will lower and somehow the liver will know it has to break down the excess...

But how it knows. Is beyond me..... It obviously gets a chemical signal but ..... No.... Dunno. Sorry.

Can anyone else fill in the gaps please?

X. G


Bumping this thread just wondered if anyone else can shed some light on my original question. Galathea has supplied a good answer was just wondering if someone could "confirm" this? :)


Logically, I would have thought that the T4 in NDT would only be converted into Reverse T3 if there was already enough Free T3 in the NDT for your needs. But if the Free T3 from the NDT was insufficient for your needs, then your body would do the conversion of Free T4 into Free T3 as you would expect.


Don't miss out on the possibilities of the T4 being processed into T4-sulphate or being glucuronidated. Both of these can then be excreted (or, I think, recycled). It is not a simple matter of all T4 being converted into either T3 or rT3.

I feel that much of the apparent understanding of rT3 which is confidently asserted may, in the end, prove to be built on poor foundations. There always seems to be too much "it seems logical that..." without sufficient hard evidence.


The Hoggle, Adding T3 to T4 may stimulate better conversion of T4 to T3 and that may be partly why FT4 drops when T4+T3 combination or NDT is taken. If rT3 is high it is often because there is too much unconverted T4. You can get rT3 tests from Genova and Blue Horizon.


I am not a medical professional and this information is not intended to be a substitute for medical guidance from your own doctor. Please check with your personal physician before applying any of these suggestions.


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