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Thyroid UK
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Is genetic test necessary for T3 conversion?

My past three blood tests have shown T3 conversion problems but my endo is requesting a genetic blood test to fully confirm this. It doesn't look like this will be covered by health insurance. Is this genetic test usually needed before a doctor will treat T3 problems?

TSH is 0.15miu/L, (2 months ago it was 0.40 so it is dropping) fT4 20.9pmol/L, fT3 was low normal at 3.2pmol/L (3.1-6.8) (2 months ago it was 3.5 so it seems to be dropping).

Symptoms include feeling tired, cold, dry skin, lack of concentration although a previous forum post here did confirm my iron is a little low which I am rectifying and a gluten intolerance was also identified. All other tests fine and optimal.


7 Replies

Can you remind members of Vit/min test results and put ranges in for the above. Will help people to comment.

Are you gluten free now? Do you take selenium?

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Silver fairy - Thanks! Selenium has never been suggested, should I take it?

Gluten intolerance has only just come up. I didn't eat much gluten previously and I have never dietary symptoms as such. I am trying to be gluten free now but too early to say if it is having other effects. Is your point that the gluten may be affecting T3 rather than the genetic disorder?

Vit D was 90 umol (I've increased dose since). Ferritin was 59 and range is 13 to 150. Liver function were normal, Hb was 118g/L. I can't find a result for the B12 at the moment but it was definitely confirmed as optimal.


The genetic test is for a polymorphism in a common DIO2 gene which causes patients to have slightly impaired T4 to T3 conversion such that they respond a little better to combined L-T3 / L-T4 therapy than those without the polymorphism. The authors state 'This polymorphism had no impact on circulating thyroid hormone levels'. Your endocrinologist does not understand the research. The polymorphism does not show up in circulating thyroid hormone levels and so the polymorphism is not responsible for your 'low' fT3 levels.

fT3 falls as fT4 rises, this is how the body keeps overall hormone levels stable, an elevated fT4 causes TSH to fall thus reducing thyroid output and type-2 deiodinase (T4 to T3 conversion). Your fT3 is reasonably normal for someone with a high fT4.

Your symptoms suggest you need more thyroid hormone. There's no point increasing your levothyroxine dose as your fT4 is already high enough, any higher and type-1 deiodinase will take over - this produces reverse T3 as well as T3. Reverse T3 blocks thyroid hormone action. It makes sense to give you some liothyronine (L-T3) to try and resolve your symptoms whilst watching out for any signs of over-treatment. This makes sense whether or not you have the polymorphism.


Thanks jimh111. Sorry just so I'm clear then the genetic test isn't needed, I could be treated without it? He had mentioned that he would treat with natural dessicated thyroid, is that what you refer to?

I should flag that I am trying to conceive and he has flagged that there is little research around treatment in pregnancy with this course.



A genetic test isn't needed to see that your conversion is poor ie low TSH, high FT4 and low FT3. Your endo could prescribe Liothyronine (T3) in addition to Levothyroxine or switch you to NDT which contains T4+T3.

The BTA doesn't support use of T3 or NDT during pregnancy so you will almost certainly be pressured to revert to Levothyroxine during the pregnancy.

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Thanks for your help, yep further things to think about if and when I get pregnant!


Hi just following up on this. Please can anyone advise why Thyroid UK says this genetic test may be useful? The more I read the more confused I get.


My endo advised that he'd prefer me to have the test because I'm going to try to conceive after this treatment.


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