Thyroid UK
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How important is it to have a RT3 blood test?

This is the one test I have not had done and am going to the doctors tomorrow. My results at the moment are:

FT3 4.6 (3.5 - 6.5)

FT4 20.10 (10.5 - 20)

TSH 0.13 (0.35 - 5.5)

I am on 150mg of Eltroxin a day and have been for 6 years. I was recently diagnosed with antibodies. I feel unwell most of the time but previous visits to my doctor have not really got me very far. Will the RT3 test tell me if I am not convering T4 to T3 properly or can anyone tell this from my test results please?

Will the doctor do the test if I ask him or will I have to have this done privately.

17 Replies

Almost certainly, he will not do it. I used to say I had never seen an NHS lab which did rT3 tests, but a while ago someone seemed to say that it is possible, sometimes.

Superficially those results look not too bad.

Have you checked out the the standard things:

vitamin B12



vitamin D

... all of which are often low in hypothyroidism.

Information about private testing here:



Many people feel best when their fT3 is in the upper half of the range (above 5 for this range). A slight increase in levothyroxine might be enough but if not a rT3 test may be useful.

Definitely get the tests Rod recommends and possibly cortisol if you can. Apparently if your cortisol levels are too high or too low this can affect how well you convert T4 to T3.

Carolyn x


Thank you Rob and Carolyn. I did have Cortisol done. Not sure which of these is the result, but it says IgA 3.67 gl, Endomysial antibody IgA level - normal. Not sure which these are. I also had:

vitamin B12 321 ng/L (180 - 900)

folate 16.70 microg/L (3 - 20)

iron 13.40 units (11-30)

ferritin 38.40 microg/L (10 - 200)

vitamin D 29.70 (<30)

The doctor has told me to take iron (1000 iu a day but I am taking 4000 iu a day)

Does this help?

So if tests are all OK, why do I still feel so bad?

Kathy x


Did you mean to write "your doctor has told you to take vitamin D?" Iron supplements are not usually measured in IU.

I reckon you may need more iron in you too. Your iron results are down at the low end of normal. I've experienced extremely low iron, and it wasn't fun. Lack of iron particularly blocks thermogeneration for some reason. Taking vitamin C and B2 at the same time help with iron absorption and utlisation.

I'm not sure that an RT3 test is that useful personally - it can be a sign that there's a problem outside the thyroid gland but it doesn't tell you anything about where the problem is.


I would say from those results that you're not converting very well - I have no idea if it's because your converting into rT3, but it's a possibility.

The T4 is right at the top of the range, the T3 is only just over mid-range. If you were converting well, the T3 would be higher in its range than the T4 is in its range. Hope that's clear, but I agree CarolynB, a cortisol would be much more useful to stick out for than an rT3. And, of course, all those vitamin and mineral tests!

Hugs, Grey

1 like

Thanks Grey

I did have cortisol and it should be one of the results I have just listed above but I've no idea which one! Anyway, I was told my cortisol was normal.


Sorry - me again. Should I be aiming for more Eltroxin or trying to get T3 added (or of course NDT)? What does anyone think please.

Kathy xx


Thank you poing. Yes the doctor told me to go and buy vit D and take 1000 iu a day but I read somewhere that this was a maintenance dose so I have been taking 4000 iu for the last two months. Thanks for the tip about Vit C and B2 with iron,

Not sure about my iron readings, very difficult to read the printout, but it looks like it says Serum Iron level 13.40 units (range 11 - 30) tell patient normal.

K x


Hi again Katkin1, I take vitamin d3 10000iu daily, one thing that is good, which the endo agreed with, also that I take 4 brazil nuts a day for my selenium. I do have high cortisol, at least I did last year, they refuse to do more testing, but that is Newcastle for you. The high cortisol can lead to high RT3, so they should test, but they won't. They have totally forgotten the hippocratic oath, or they don't care about it in the first place.

My iron is terrible, because even at the age of 60, I am still a normal menstruating woman, thus showing that one size does not fit all, in any way!!! This can be another reason for high RT3, B6 B12, zinc, dhea, chromium, saliva cortisol 24hr testing, the 2 antibodies, iodine zinc and chromium.

These tests should all be done, if RT3 is found to be too high, I am getting mine done in Crete next year, too expensive here.

Good luck with this Veronica


Hi katkin

Your b12 is on the low side.... I read an optimal result should be between 600-800 ish. So you wud be ok to supplement this with a sublingual 1000mcg tab ( check out AMazon). Your ferritin is optimal between 60-90 ish . I got my numbers from the book Stop The Thyroid Madness. Just check to ensure i have my numbers right (foggy brain)Your iron supplement will sort this eventually. Don't take it close to your thyroid meds or near having tea or coffee though. Taking iron with orange juice ( or vit c )will help absorption.hope this helps a bit?

Karen x


Thank you for your help Karen. I have ordered some B12 from Amazon.

Kathy x


Hi Kathy

Just a pointer, if you want to reply to someone in particular, you have to make sure to click on 'reply to this' otherwise they will not get an email letting them know that you have responded.


x :)


Thank you Louise. I didn't realise this.

Kathy xx


This may be helpful about reverse T3 levels. For one thing, T4 is going to turn into T3 or RT3 and I've read this process occurs in the liver.

> Reverse T3 (RT3) is what is made when the "wrong"

> iodine atom is removed from T4, it's a "mirror

> image" molecule to T3 and is not bio-active. This

> in itself is not a problem, the problem is that in

> excess it "fits into the T3 receptors" and gets

> stuck there blocking the action of T3 on the body.

> This means that your body doesn't respond

> properly to T3 leading to hypothyroid symptoms

> despite a normal TSH and normal T3 and T4 levels

> in the body. This is what we refer to as "Tissue

> Resistance to Thyroid Hormone". Doctors will tell

> you this is very rare, this is because they don't

> look for it!!



Thank you Heloise. Considering I am now seeing a doctor who thinks that if we get the readings right for TSH, FT4 and FT3, we can then look at the other problems separately, ie tiredness/exhaustion, fibromyalgia etc and treat them separately, I don't think I need worry too much about RT3 as he doesn' even seem to understand the basics.

I feel like all the fight is gradually going out of me as I thought that this doctor, as head of the prcatice, would be the most informed about thyroid problems. How wrong can you be?


1 like

I know this post is old, but if your still out there how are you? You didn't post your cortisol levels anywhere, The antibiodies test you referred to is to check for celiac disease not cortisol.. Your Tsh Was Way to Low, no wonder you feel bad! & Your T4 too high, can cause you hyperthyroid symptoms. You have Low Vitamin D, Need Iron, Your on lower end. I would ask doctor why your TSh is so low if it still is. Also get your cortisol checked request it if you haven't already. I would get tested every 3 months, get copies of labs and post new results. I hope your feeling better.


HI Katkin1, I am on 100/125mcg T4 and 50mcg T3 daily, I self- medicate with T3, which I bought in Crete, as the docs won't prescribe it. My FRee T3 level was only 3.7, and after 3 months on T3, it is now 7.9, the scale that I go by is 4-8. Proof of the pudding.

The doc will not do RT3 testing, so I will get it done in Crete, much cheaper next year, for the mo, I am doing ok.

The only thing that I want tested now is most importantly Rt3, because there can be lots of Free T3 circulating, but the T4 is producing too much RT3, thus blocking the T3 from going intracellular for release of energy, or so I understand.

The ratio is a bit hard to understand, some say 10:1 T3 to RT3, and some say 1:1. I would go with the 10:1, otherwise you would be RT3 dominant.

A good article to plough through is

Take care



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