Why does T3 lower T4?

[Zebra5]

Please could someone explain why taking Liothyronine when using T4/T3 combination therapy lowers T4? Does this always happen? Thanks for your help.

[greygoose]

Good question. Wish someone knew the answer. It's been discussed on here many times, and there are lots of theories, but no-one really knows.

I think it always happens, yes, to a greater or lesser extent.

[Patriciawoodward in reply to greygoose]

Thyroid hormone output is regulated by a negative feedback mechanism. If you have a thyroid gland this mechanism will operate. Since T3 is produced from T4, you will not need to produce as much of your natural T4 when taking exogenous hormones as LT4 conversion to T3 is triggering the negative feedback loop.If you were previously on LT4 only, then changed to combination treatment you will have a reduced LT4 script to take account of the negative feedback since you are increasing LT3. This will clearly show in your blood as a lower free T4. And hopefully increased free T3 to clear your symptoms.

[greygoose in reply to Patriciawoodward]

If only it were as simple as that. If you are on a full replacement dose of thyroid hormone, the negative feed-back loop won't have any effect. It only affects the FT4 level if it's a workiing thyroid that is producing the T4. It won't have any effect on exogenous T4.

Once you are taking T3, your TSH is going to be very low, if not entirely suppressed. So, how does it have any effect on anything? Not on the thyroid, not on conversion.

Yes, the levo dose is often reduced, but even if it isn't, the FT4 level will drop when you start taking T3. So, I don't really understand your theory, here. It just doesn't apply.

[tattybogle]

i don't know the answer, i don't think anyone does in any scientifically proven sort of way , but it's always seemed quite logical to me that this will happen. We have deiodinases that convert T4 into either T3 or reverse T3 , how much and how fast they convert T4 into either T3 or reverse T3, is dependant on feedback from the TSH level and the levels of T4 and levels of T3. We also have deiodinases that get rid of T3 by turning it into T2.. again how fast or how slow the T3 is converted to T2 is dependant on levels of T4 and T3 and TSH.

Seems to me, if you keep the same T4 dose and add some T3, then some fiendishly complicated deiodinase regulating action is quite likely to come to the conclusion that since your overall thyroid hormone level (T4+T3)has increased , it now thinks you don't need as much T4 as a store any more and will either convert more of the T4 to reverse T3 than it was doing previously, or it will turn down your own thyroids production of T4 even more than it already was ( if you still have any functioning thyroidal production of T4 left at all ) or both .

But try as i might i seem unable to remember clearly which deiodinase does what under which circumstances... so i put it all in the 'too complicated' box and think about something else less complicated instead.... like whether i want peanut butter or marmalade on toast for breakfast.

I'm going with peanut butter.

[greygoose in reply to tattybogle]

If more T4 was converted to rT3 when you add T3 to T4, it could be easily proved by monitoring rT3 levels. So, why has no-one ever done that? Of course, we do know that if you take too much T3, more T4 will convert to rT3 to stop you 'going hyper', to coin a phrase. But, even on modest doses of T3 will lower the level of rT3, so I doubt it's that. Personnally, I go with the theory that, as you don't need as much T4 when taking T3, more of it will be excreted. But my idea was shot down and poo-pooed by certain members on here when I proposed it!🤣 🤣🤣 But, it still makes sense to me - in a way... But, like all theories, you can always say, yes, but what about... In thise case, I would say the 'what about' is about those that need highish levels of FT4 to feel well, even when taking T3. So...

[tattybogle in reply to greygoose]

well i'm not going to pop poo your 'poop it out' theory

[greygoose in reply to tattybogle]

I suppose you could put it like that, yes... Think it's more likely to be in urine, though.

[tattybogle in reply to greygoose]

yes me to, but i just wanted and excuse to write 'poo' again.

[greygoose in reply to tattybogle]

I see... How old are you? 12? lol

[tattybogle in reply to greygoose]

9 and 3 quarters (i like to stay under the age of criminal responsibility)

[Zebra5]

greygoose tattybogle Patriciawoodward Thank you very much for replying. I have come down with Covid so will be back when better to respond properly. Hope you have a Happy Easter!

[Zebra5]

greygoose tattybogle Patriciawoodward I am better now and back. Many thanks for your replies which I have now read properly. It is indeed complicated and I understand tattybogle why you turn to less complicated questions such as marmalade or peanut butter ...

🙂

[tattybogle in reply to Zebra5]

Glad you're still with us .. today's a marmalade day