Impact of levothyroxine on thyroid direct production of T3

[Curious123]

My understanding is the thyroid produces both T4 and T3 although the majority of our T3 comes from conversion of T4 to T3. If we are taking thyroxine only (T4), which is lowering TSH, would it effectively cause a decrease in the amount of T3 being produced directly by the thyroid gland in the scenario where the thyroid gland is still producing a portion of T3 and T4?

[greygoose]

In a word: yes.

Once the TSH gets below a certain point the thyroid more or less goes to sleep and stops producing any hormone. Plus, you will be missing the conversion of T4 to T3 that happens within the thyroid itself. So, not surprising most people on T4 mono-therapy have low T3.

[Curious123 in reply to greygoose]

Thanks. Also then explains why lots have weight issues if T3 is the one impacting metabolism. I’ve decreased my dose and not tanked my T3 and T4 so 🤞🏻I can keep reducing and the iron infusion will help improve T4 to T3 conversion.

[greygoose in reply to Curious123]

Well, it might. Then again, it might not. There are so many causes of poor conversion, it's not all down to nutrients.

[Curious123 in reply to greygoose]

Yes, but now I can at least see if more optimal iron status makes a difference for me. I know pre any significant thyroid issues my thyroid always went out when I became iron deficient.

[Mlinde in reply to greygoose]

It all depends on whether you convert T4 to T3 properly, many don't. This is really important because T3 is the active ingredient that helps the body's cells produce energy. It explains why lack of T3 can effect the body's functioning so drastically.

[greygoose in reply to Mlinde]

I'm sorry, what depends on how well you convert? We weren't talking about conversion, were we?

T3 is the active hormone, yes. And, no, not everybody converts very well. But that has nothing to do with T3 production within the thyroid.

T3 is needed by every single cell in your body to function correctly, not just to produce energy but to do what they are supposed to do - which varies from cell to cell. Without adequate T3, the whole body ceases to function optimally.

[Mlinde in reply to greygoose]

I never said it did.

[Mednews4 in reply to greygoose]

so would it be possible to only take t3 if you have hashimotos? If not converting from t4 to t3. My tsh is now quite low after introducing t3. T3 and t4 were mid range last testing. Why not just take t3 if it’s lower range?

[greygoose in reply to Mednews4]

T3 mono-therapy should be a last resort because taking T4 with it gives you a certain security, a back-up, should there be any sort of problem with your T3 supply - which, in these uncertain days could easily happen. Taking T3 only - especially if you have to take a high dose - is a risk.

But it's a risk I was forced to take because I couldn't tolerate T4, it made me ill. It had nothing to do with having Hashi's - which I have, nor with conversion, I converted very well, and it was nothing to do with the brand - I couldn't tolerate any brand. And, in the beginning I needed a very high dose of T3 - 225 mcg at one point. It's now come down to 75 mcg, but even so, it's a very precarious way to live. Doctors don't want to prescribe that much T3, so you're on your own. And buying your own can be expensive - if you can find a provider, that is.

But apart from all that, there's no reason why you shouldn't take T3 only.

[Tea7light]

When I was allowed on T3 the endocrinologist told me I needed to take T4 in tandem, as in the future it would hopefully long term help keep Osteoporosis & heart problems at bay. It was not advisable to solely take T3. (But I realise we have colleagues on this forum who take T3 for reasons other than thyroid problems.)

As a normal thyroid works with T1, T2, T3 & T4. The proportions of the latter 2 elements are known, but the smaller quantities of the former are unknown. But they are there for a reason?

I had problems on solely T4, when on T3 + T4, my brain fogs faded, and I could converse once again.