Thyroid UK
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An update to recent post 'Is this a conversion issue'?

I got some advice earlier in the week about my latest blood results which looked like I was not converting enough T4 into T3 with my T4 just over the average range but T3 in the lower quartile.

I spoke to a nurse on the phone quite in-depth when trying to make an appointment for other blood tests to rule out other causes of my extreme fatigue, muscle pains, pins and needles, weakness etc. Interestingly she can see from my notes that free t3 has been measured before so now I have a direct comparison of my t3 on 175mcg and 200mcg of levothyroxine.

Results April 2017 (200mcg Levothyroxine)

TOTAL THYROXINE(T4) - 143 nmol/L ( 59 - 154 )



FREE THYROXINE - *22.4pmol/l ( 12.0 - 22.0


FREE T3 - 3.9 pmol/L ( 3.1 - 6.8 )

Results March 2016 (175 mcg Levothyroxine)

FREE THYROXINE - 12.4pmol/l ( 12.0 - 22.0


FREE T3 - 4.6 pmol/L ( 3.1 - 6.8 )

Sorry, these results are not full as they were just given over the phone and I only asked how my t3 had changed over time.

So even though I felt very unwell at 175mcg and was very low and suicidal enough for GP to raise my levo up as a trial to see if my symptoms improved, it does appear that I was able to convert better T3 at this dose so not necessarily a conversion issue as I first thought? Maybe this is, as somebody else suggested, excess t4 being converted into rT3 and so not biologically viable.

So I feel unwell on a dose of 175mcg, and feel even worse on a dose of 200mcg as less t4 to t3 conversion is happening at this dose.

The nurse has suggested a full appointment with a GP to discuss all my symptoms with an open mind as admittedly I went in for my recent one totally focused on the thyroid dose and convinced my results would show hypothyroid so did not consider other tests. Are there any I should ask for apart from b12, folate, ferritin? I'm still hoping there is something the GP can easily spot and work to fix before I go down the route of self-medicating.

Thank you all for your sage advice, as always

14 Replies

From these figures, it looks like the higher T4 dose is actually counterproductive, because at the lower dose your FT4/FT3 ratio is 2.7, whereas at the higher dose it is 5.7. And the FT3 barely changed with both doses. This appears to show as is well known, that too much T4 can actually work against the patient by both promoting rT3 production and actually inhibiting T3 production. The body sees the excess T4 as toxic and removes it by the rT3 route. You don't seem on the face of it to be a poor converter at lower T4 levels.


Yes, I can see that now. Definitely do not want to supplement with T3 either if a conversion issue is not a stake. Gp did recommend last week to reduce levo from 200mcg to 175mcg to bring me back into normal t4 range, which is was reluctant to do whilst having low quartile t3 and still feeling so unwell but I have reduced the dose as of this morning as can see the higher dose is not helping at all.

Are there any other tests you can think of to ask for to get to the bottom of symptoms? I had total thyroidectomy about 4 years ago.


This is very interesting. So, at what point does the T4 become excess? And what do you do if, at the point at which it becomes excess, the T3 is still too low to make the person well?


There should be a sweet spot always where either T4 on its own or combo if needed is optimal for the individual. Rise above that and rT3 starts rising fast to get rid of the excess T4 and worse, you can inhibit T3 production. So high rT3 should indicate overdosing.

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Well, yes, l know that, but that's not what l asked. At what level does the body consider T4 to be in excess and start converting it to rT3, how high can it get before that happens? Or is it an individualmthing? And doesn't that rather make a mockery of the FT4 range?


It's an individual thing. And FT4 isn't a useful parameter for estimating satisfactory treatment. FT3 is.


Please stop talking to me as if I just stepped off the boat! I am well aware that the FT4 is not a useful parameter for estimating etc.

I'll take it that you either haven't understood my question, or don't know the answer.


I don't know how I could say more. The individual body decides if T4 is in excess through its own individual makeup genetically. By that I mean the original balance between direct T3 supply by the thyroid, and T4 conversion in the body, to give together the optimal FT3 for that individual. This may be high in range or low in health, depending on the genetic response of the individual to T3. So giving T4 when there is no thyroid will only allow conversion in the absence of the thyroid, but the point at which an individual takes so much T4 that the body calls it excess and drains off the extra T4 as rT3 is strictly an individual experience, which in turn depends on when it feels the FT3 is optimal. Try to go higher with T4 and either overdosing occurs from too much T3 or if the body can't do any more conversion, then rT3 is made.


Thank you so much. That answers my question.

So, I take it that the only way to know when you've reached that point is to test the rT3. Understood.


Yes, looking at rT3 is fine, but beware that this can also rise with a severe non thyroidal illness, when the body goes into partial shutdown mode and switches production of T3 to rT3 from T4.


Yes, I know. :)


Did you allow a gap of 24 hours between your last combination dose and the test?

Test always to be the very earliest possible.

Procedure for tests. Allow a gap of 24 hours between last dose and the test and take afterwards.


I do not always have the earliest test of the day, but it always before midday, always fasting and always without a morning dose of Levo so at least 24 hours from the previous dose


The reason Bumble2017 is that the TSH is highest very early morning and then begins to drop during the day, so that can mean not getting an increase you may need as GP is happy with the TSH. The aim is a TSH of 1 or lower, some of us need it suppressed.


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