Thyroid UK

How do you know if you are converting T4 in to T3 properly

I am trying to learn more about conversion . I recently had my free T3 tested privately as my labs would not do it .The NHS were suppose to do Free T4 on the same day but ignored my Gp ,they only did TSH .

I know this isn't accurate but just as an example if my Free T3 was. 3.9 and my free T4 was 19.1 would that indicate

i'm converting properly or not ?

The ranges for T3 were 3.1-6,8 . ---- for T4 they were 12.0-22.0

My TSH was 0.28

The figures for the Free T4 were from February so I know I cannot take any these figures as accurate but if my Free T4 was the same now as then would it show i'm converting properly or not .?

Thank you for any replies .

5 Replies

Ideally both FT4 and FT3 will be in the top 75% of range. Your FT4 is pretty much there but your FT3 is low in range which may indicate a conversion problem if you have been on Levothyroxine a long time.

You could ask your GP to prescribe Liothyronine (T3) to see whether this will raise your FT3 and resolve any ongoing symptoms.


Hi Clutter am I right in believing that being on Levo for so long like myself (20 years) can stop conversion of T3. I saw this also with Dr Holtorf yesterday and it got me thinking. I have recently started T3. I did it myself with cynomel and was feeling ok but when I reached 50mcgs was too high. I had gradually stopped the T4 and my tsh went up to 29 from 0.07. So my doctor put me back on Levo 100mcgs which I have been taking. My Endo has been trialling me on the NHS T3 for 3 months 20mcgs and Levo 100mcgs. She has now told me to drop the Levo to 50mcgs and up the T3 to 30mcgs. I do find it weak the nhs T3 (weaker than cynomel) because I have ran out and awaiting prescription have taken 2 cynomel over the last two days and I feel the difference like a warmth in my chest. Still have all the symptoms depression weight gain aching joints etc etc. Mentioned about optimized bloods ie B12 Ferritin and Folate got lost in conversation, I am taking Vit D as was Low 23 now 78 doctor has given me. I have started Magnesium but found it can upset bowels, but will carry on as best as I can. These are my latest bloods TSH 0.07 T4 8.5 sorry do not have ranges yet and T3 4.6 (3.00 - 6.00) did not take the T3 in the morning of bloods. I just want to feel well. Any comments would be appreciated.


Thank you Clutter I have been on T4 for over 10 years . I will see if my Gp is willing to prescribe it for me .


This is the second time recently that I have read being on T4 for a long time can cause problems with conversion .Certainly will be discussing T3 when I next see Gp .I am having to wait a month as they are keeping an eye on my red cell blood count (high) re testing in another 3 weeks .



clearly your not converting t4 into t3 but that could simply be because like so many hypothyroids your ferritin and possibly folate are way too low

ferritin and folate must be at least halfway up in their ranges or you cannot convert t4

do not let your gp say they are "in range" because that means squat


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