Can levo cause low FT3?

[Opal79]

Hi I am hypothyroid with Hashimoto’s, suspect both for decades but never tested/ treated until 2 years ago. I also took private tests at the same time and whilst my TSH and FT4 were both around 8, my FT3 was optimal.

I was put on levo by GP and titrated to 100mcg but last autumn got worse, (also privately dx with P.A last year and self injecting B12 eod since).

I do think I also need more thyroid meds in winter compared with summer however since being on levo my FT3 has been very low end of normal even when getting TSH to just under 1 (FT4 got abit high today normal tho).

Currently consulting a private endo who will be giving me NDT but my question is, why would FT3 be ok at time of dx and not now? Could the levo alone have caused this lack of conversion when it was once optimal? Thankyou

[Lavender-Blue]

I understand T4 to T3 conversion becomes less upon increase of T4, as TSH reduces, this is what happens.

I have just learned this from Paul Robinson's book.

I think if you do a Google search regarding the above with Paul's name in the search, you should find his blog relating to this.

Edit....I think this is the link to Paul's blog:

paulrobinsonthyroid.com/mor...

[Opal79 in reply to Lavender-Blue]

Thankyou so much for your reply and the link, I'll have a read. So could this mean that the levo could have done this? Despite when TSH/FT4 bad I was still converting then and not now? Thanks again!

[tattybogle]

a higher TSH level does a couple of things:

1) it increases the ratio of T3 to T4 that the thyroid produces, (so it produces relatively more T3 than usual)

2) it also increases the rate of conversion of T4 to T3 that happens around the body.

both the above seen to be a form of emergency 'T3 boost'.

So , at diagnosis ( with high TSH) it is common to have relatively good looking fT3 levels ..... because the system is trying it's best to maintain T3 levels for as long as possible when thyroid gland is starting to fail and T4 level is falling .

then , when T4 level is increased by taking levo , the TSH level lowers in response to higher T4 level.

When TSH level is lower, both these emergency T3 boost functions are turned down / off .

[Opal79 in reply to tattybogle]

Oh thanks so much for explaining! That makes so much sense. I wasn't sure if it was because of my P.A and liver being unable to convert but after a year of injections my thyroid is still worsening. So my thyroid likely could have been failing quite recently then and not for years as id previously thought? Thanks so much again

[tattybogle in reply to Opal79]

it's hard to know what is going on or why . the only thing we really know is 'things do change'... i had reasonably ok T3 levels for 20 yrs from levo only , then for no apparent reason ...i didn't. So it's just a constant case of 'adapt and survive' isn't it .

[Opal79 in reply to tattybogle]

Thanks very much, yes it's mind boggling trying to understand this stuff! I'm sorry you managed ok for so long then found things worse, I hope your T3 is better now and things are going as well as poss?

It really is adapt or survive, yes! And so frustrating that most doctors don't really help either. Thanks so much again