Thyroid UK

T4 to T3 conversion – blood tests versus urine tests

I was recently diagnosed with hypothyroidism (autoimmune, Hashimoto). A while ago I wrote a blog about how I was diagnosed in Spain and subsequently denied a diagnose in the UK and ended up having to go private to get help…

I’m still new to all this world and still trying to learn as much as I can, so I don’t feel very qualified to impart words of wisdom, however I thought I’d share the following with this forum in case it helps or inspires others who like me are still trying to get to grips with this. This may very well be "old news" for the veterans, so apologies for those who may feel I'm just stating the obvious.

The private Dr. I went to after being denied a diagnose by my GP took a good look at the blood tests I had had done in Spain and confirmed that my diagnose is “obvious” given the elevated antibodies and my symptoms. He then proceeded to order two tests: a blood test for full thyroid panel and a 24 hour urine test for the same (this is a test where you collect your whole urine for 24 hours, which poses some logistical challenges!). I had both done on the same day and this was before starting medication.

Here’s what the results looked like:

Blood tests

TSH3.73 (range 0.3 – 4.5)

FT33.06 (range 2.1-4.2)

FT41.20 (range 0.7-1.8)

+elevated anti-thyroglobulin antibodies

(So, apart from the antibodies, all nicely “in range”… my tests in Spain months ago had had a much higher TSH and much lower FT4)

24 hr. Urine test

T3 445 pmol/24hr. (range 800-2500)

T42130 pmol/24hr (range 550-3160)

(So T3 is desperately low –half of what it should be- and T4 is more or less OK)

The Dr. explained the blood test gives a general idea of how much hormone is circulating in the blood at a particular point in time, but not necessarily how much is reaching the cells. The urine test gives a better idea of the level of thyroid hormone that reaches the tissues that need it.

He explained my urine test clearly points to a conversion problem, which explains why I have clear symptoms, in spite of the fact that my blood test “looks good” and that according to it I’d be classified, if anything, as “subclinical” (ie. symptom-free) by an "institutionalised" GP or Endo who would look no further... The urine is telling a very different story, my cells are only getting 50% of the T3 they should be getting, so I am significantly deficient in it, not just a wee bit... Standard T4 medication would not work for me at all, so he is giving me Euthyral which is a synthetic T4/T3 combination on a 4 to 1 ratio.

With my same blood test results, someone without a conversion problem would probably be feeling just about alright with no symptoms or hardly any… however due to the conversion problem I've started to struggle already.

The whole “conversion” issue is so complex. As far as I can work out the body needs an adequate level of iron, selenium and zinc for the process (all of which I am deficient in and I am now supplementing) as well as a healthy balanced intestinal flora, a detoxed liver in immaculately perfect condition and a mind in peaceful equilibrium free of stress, worries or concerns…

Whatever the case, the reason I wanted to post this is that for so many people who are being denied a diagnose and told by doctors that their symptoms have nothing to do with the thyroid because the blood test results look good… it seems to me that there is a test that can be done, the 24 hr. urine test, that should enlighten the doctors and make them understand that something is not right and that it is not all in the patients “head”!! Same goes for so many who don’t do well on T4 only medication and are denied T3. This test could be used to establish whether there is any point in giving T4 to someone who cannot convert it into T3…

All the best to everyone!

27 Replies


The conversion issue is probably due to "Non-Thyroidal Illness Syndrome"

Deiodinase Enzyme D3 is activated, which converts Thyroxine (T4) to Reverse Triiodothyronine (rT3), which both inhibits Deiodinase Enzyme D1 and blocks the cellular FT3 receptor sites.

The more L-T4 (Levothyroxine) Monotherapy thrown at you, the more rT3 is produced and the greater the inhibition of Deiodinase Enzyme D1 which in the body is responsible for producing Triiodothyronine (T3) from Thyroxine (T4)

Do not accept combination therapy that leaves you with a Free Triiodothyronine (FT3) of less than 5.5 pmol/l: The FT3 danger-zone for a host of conditions including Dilated Cardiomyopathy (Congestive Heart Failure) starts at 5.4 pmol/l and the risk increases as FT3 gets lower.

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Hi Mighty Mouse, I am trying to convince my endo that this is my problem but he needs 'evidence'. Can you give me the reference to the article/s that this information comes from? I would be very grateful, thank you. Lisa



Understanding Local Control of Thyroid Hormones:

(Deiodinases Function and Activity)

There is a poorly handled explanation of how pituitary FT3 causes Central HYpothyroidism and an inaccuracy re FT3 not passing the blood/brain barrier (It passes at a ratio of 10% of the blood serum concentration), but the rest is fairly accurate and solid stuff.


thanks Mighty_mouse. Unfortunately I can't open the link. Is it possible to send it in another form? thanks Lisa


I would agree with mighty_mouse. I can't see the point in giving you T4 if you can't convert it. If it is an rT3 problem, the problem is just going to continue. (Why didn't he test the rT3?) You need to clear the rT3 from the receptor cells not add more to it. I would have thought that T3 only would have been more appropriate.

As to the 24 hr urine test, how easy is that to get on the NHS? And if it were done, would NHS doctors know what they were looking at?!?

Hugs, Grey


The reverse T3 test costs £122 privately- maybe in the NHS it could cost more, for all I know. NICE have been quoted at me when I push for tests with my NHS GP -and I don't get very far there, for sure!

Money talks- or rather doesn't in our favour, sadly.

Very helpful post Chiquitita, btw. I'll bear your points in mind if I need ongoing rT3 work.


NICE has issued no thyroid guidelines apart from one re a specific eye issue.

However, they have become responsible for the former CKS (Clinical Knowledge Summary) pages - which now say "NICE" somewhere on them. But these are absolutely NOT proper NICE guidelines and their maintenance has been somewhat poor. (Even if they were any good to begin with - which, for thyroid issues, they weren't. In my opinion.)



Sadly, the Dr concerned has now left the practice so I can't ask further.

He was supportive, but never stepped too far from a guideline approach.

He did get some indepth general blood testing done a while back, though.

He promised to pursue my 'Hypo' case later -but skipped away when I wasn't looking :~/

I am doing private tests but that is only advisory - so I hope something screams out of the page to present to GP - or I'll be back to square one.


I put a reply to this, but it is showing below... thanks for your feedback and comments!


That is MOST interesting. It reveals such a lot that the blood does not. I do hope that the combination therapy will work for you. If not, the T3 only route may well prove to be your best option.

It's very interesting that a short while ago I did a comparison with several different people's blood tests and the ones who were saying they felt better had a ratio of T4:T3 of 4:1 or even 3.5:1 so it shows how important that ratio is! my own ratio is only 6:1 and my T3 falls well below the level mentioned by mighty_mouse at 4.1, which probably explains why my doctor says I have an enlarged heart! That was on 125, and I had been on 100 of Levo for 7 years. Wonder how low it was then!

I do hope that the care you are having now will help you to get better. We all deserve that.

Marie XXX


Get that FT3 above 5.6 pmol/l.

A healthy FT4:FT3 ratio is 3:1


That's a beautifully written and very clear piece C which illustrates the problems so many of us have run into in getting diagnosed - very much where I was for many years although my free T3 dived once or twice, but always recovered - leaving my GP able to repeat the 'normal' mantra.

The use of a 24hr urine test to detect whether or not the thyroid hormone is being converted is new to me. I'm another that would love to hear details - my endo suggested that there was no practical test available to detect conversion issues.

Your description of the problem of T4 monotherapy MM is likewise impressively clear and covers a lot of ground concisely - especially on rT3. I'd not seen mention before of a specific FT 3 threshold below which cardiac issues could be expected, and only seen that picture spread out between multiple reference souces.

I wonder if it's written up in a single authoritative reference anywhere that would function as a one stop source to point my own (interested, pretty open, but inevitably conservative) and others' endos to?

Thank you.



thank you for this - I too would like to know more x


Hi again!

Thanks so much for the comments and feedback. I wish there was a way or replying to all who’ve written comments at the same time, but I think there isn’t, so I’ll post my generic reply to all comments here.

My Reverse T3 was tested (as part of the blood test) and it came up as:

0.16 (range 0.14-0.54)

So at the lower end of the “normal” range... not sure conversion of T4 into rT3 (instead of T3) is the issue for me, but I have found the above information from Mighty Mouse most interesting and I will definitely ask the question on my follow up appointment coming up in a few months.

I did ask my Dr. about why starting on combination T4/T3 medication instead of just T3 medication.

The reasoning behind this was (I’ll try to explain best I can): converting T4 into T3 properly is something that our bodies are designed to do and in an ideal world should be able to do... So trying to get to the bottom of why it is not doing it is important. The Dr. thought starting directly on T3 only medication, in my particular case (not generalising!) would not be ideal, as he thinks there is a lot that can be done in my case to improve the conversion problem and this should be done in the first instance. He went on to say that he has other patients who he DID put on T3 only medication from the start, so he is not anti it, just discriminating. In his own words “I’m not throwing in the towel with you just yet, I want you to convert, let’s work at it”. He suggested starting me on a mellow dose of combination T4/T3 medication first, whilst at the same time working on improving the conversion problem, review everything again in a few months and decide course of action then, according to what the tests say and (most importantly!) how I am feeling.

In his experience, many patients are able to significantly improve conversion through dietary changes, supplements and lifestyle changes (stress is a key issue…), to the point where they cease to need T3 and can reduce medication… others simply don’t… and they need T3 only medication, they should get it and they do really well on it.

I found it interesting he mentioned that even patients who generally convert well and do well on T4 only medication can go through challenges such as a bereavement, redundancy, divorce, having to care for an aging relative etc. and suddenly “shut down” and cease to convert… The mind-body and stress connection is so powerful. He said in these scenarios he has been known to switch people to different medication (T4/T3 or even just T3) temporarily and this has worked well..

The whole conversion topic is so complex and there are so many reasons why things could go wrong… A significant part of the process (I think between 20 and 30 %) takes place in the gastro intestinal tract and here is where the main problem seems to be for me (apparently this is very common). Other blood test results, urine test results and a stool test result show that my intestinal flora is very unbalanced, way too alkaline, due to a too acidic (protein rich diet). In addition to this, tests also show that I am severely allergic to gluten (in spite of no digestive symptoms when eating it). All of this would account for a significant part of my conversion problem, apparently. And to the development of autoimmunity problems (Hashimoto).

So with all this in mind, the plan of action I was recommended is to start on the low combination T4/T3 medication, adrenal medication (pregnenolone), supplements for all the millions of things I am deficient in due probably to my bad intestinal flora (the list is endless, B12, Vitamin D, Vitamin A, Co-Enzime Q10, Selenium, Magnesium, Potasium, Zinc and I am probably missing a few), introduce radical dietary changes adding more leafy vegetables (which are most alkaline), go gluten free (which is a big deal… at least for me), continuing to eat good protein but take it a bit easy, plenty of low fructose fruits and adding a lot (a lot!) of healthy oils to improve the gut mucosa (apparently this is very important for absorption and conversion!) so plenty of olive oil, hemp oil, flaxeed oil…

It is very early days, but I am already noticing an improvement, so I think I am on the right track. The first two days on medication were awful, though, I felt completely wired up and just could not sleep at all. I guess my body was in complete shock at suddenly getting some T3... But after those two days, I started to sleep well, and started to feel better. I am happy to report back next time I have a review of the whole thing and let all know if the conversion level has improved as a result of all of this… time will tell.

As to getting the 24 hour urine test on the NHS… I guess the chances of that happening are 0... And doctors probably wouldn’t have a clue! Both things are a scandal.

Finally, here’s an article I enjoyed reading about the conversion topic


Bye for now!

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Your treatment sounds similar to mine (pregnenolone, DHEA and other supplements, good gf diet) except that my doctor is more in favour of protein and I was on T4 only until recently. The thing that made the most difference to me was adding in some T3 (all the other stuff didn't really make a dent in my condition until T3 was prescribed) but hopefully now everything else will get a chance to work.

Gf is a pain at first but gets a lot easier. The eating out part is hard - you can never be sure they've really taken any notice - but in terms of finding acceptable (non-junk) substitutes for the usual gluten products it isn't too hard. It just takes time. Don't know if you're already using it but the Health Unlocked gf group is very helpful too.

How did you find your doctor (recommended here or did you find him yourself or...)?


I wonder if you would share where / who your endo is? I have just about given up with GP/endos and am trying to do it myself although not totally succesfully - your endo sounds like just the endo I need - could you pm me pelase?


me too - although happy where I am - it's always good to have another in the back pocket so to speak!


Hi C, i was directed to your thread earlier today and have found the information you have provided extremely valuable as a newbie to all of this you made it easy for me to understand, Your endo sounds amazing could you possibly pm me with his details?

I hope you continue to feel better :-))

Many Thanks Angel :-)


Hi chiquitita

Do you have the name of the Dr/specialist who recommended the 24 hour urine test?




I think T4 to T3 conversion is one of the most important issues re the treatment of hypothyroidism patients.

The responses from everyone here are just brilliant. I have been searching for answers for a very long time.

Like Marram, I too have an enlarged heart. Mine was discovered 25 years ago when I was first diagnosed with Hashimoto’s thyroiditis. At this time, I was told that it was due to not being adequately replaced with thyroxine.

I found the following article re "Non-Thyroidal Illness Syndrome" and I have downloaded the PDF and I am going to print it out:-

My sincere thanks to everyone.



The third enzyme, ID3, deiodinates the inner thyronine ring, converting T4 to rT3 and T3 to T2. It’s activity in liver is up-regulated in NTIS.



Can you please PM me with the Private Doctor who confirmed your situation as my bloods are worse than yours, I have high tpo, and extremely high tgab but I have been told I am not ill enough to warrant medication.

I need to get sorted so any help is welcome. thanks.


I have just stumbled across your post and it was like reading my own story!! I have struggled with symptoms since being diagnosed with Hypo and put on T4 / Levo nearly three years ago. Despite repeated visits to my NHS GP I also got the NICE guidelines and "our hands are tied" reply most of the time when asking for extra tests......including an ultrasound on my thyroid! Not one GP that I saw at my surgery even physically examined my thyroid.

My initial visit to my GP was because I had various symptoms suggesting PCOS but after blood tests was diagnosed as Hypo and the PCOS issue was ignored thereafter as blood tests results suggested I didn't have it. After two years I was still experiencing, not only Hypo symptoms, but PCOS symptoms too and pushed for my GP to send me for an ultrasound on my ovaries. They did agree to that and I went for the scan on NHS only to find that I did indeed has PCOS and they also discovered a blokced fallopian tube (hydrosalpinx). I'm fortunate enough to have private BUPA cover through work so was seen fairly quickly to have ovarian drilling performed and potentially my fallopian tube removed (however, when they actually did the surgery in Sept 2014, the blockage had gone so I still have two fallopian tubes...hoorah!). During the course of the treatment the private consultant needed to do some blood tests and, as I was moaning about the NHS not performing test for FT3, FT4 & RT3, the consultant got those done too!!!

I waited to fully recover from the op and see if the ovarian drilling had any knock on effect / relief to my thyroid as I know they can be linked but never really got the test results confirmed (high / low / normal etc). After a few months I contacted a private Endo to find out if they would be prepared to prescribe NDT if ultimately needed following any further tests and was told that they would not so, I have recently been to see a Dr who was on the recommended Dr's list in the pack I got from Thyroid UK and it has been like a breath of fresh air to speak with a Dr who is sympathetic and knows what you're talking about and has experience and CAN do something to help. I've had to do this privately and pay for it (not covered by, let down by NHS which I have no choice whether I pay for or not, its stolen from my wages every month, and let down by private medical insurance too!!). I saw the Dr last week for a follow up after having various bloods and 24hr urine test and it has been confirmed that I have Hashimotos Thyroiditis as my antibodies are skyhigh! My T3 levels in my blood were within range but low in urine test. My ferritin, SHBG, DHEA & Cortisol are all low so I am now waiting for appointment with a private Endo for further investigations to check for Addisons disease etc before introducing T3 treatment to my T4 (NHS prescribe 75mcg but I upped this to 100 before xmas 2014 and the private Dr has just told me to increase to 150mcg as I am mid range (2.56mU/L) on this by European guidelines (as UK is crazy!) and she likes to see her patients at around 1.0mU/L as most people feel ok at this level.

I do feel totally let down and frustrated by the NHS but feel so fortunate that I've been in a position to get private treatment. Lets hope that BUPA cover this next visit to Endo though as I might have to remortgage otherwise!!! :/


I read with interest your Urine Test Results, having just received mine. I am on 100 mcg Thyroxine, and have been nagging GP for 2.5 years that I am still not right, hence going private. (wonder where I have heard that before). My 24 hour Urine test results when reading Chiquititas have left me feeling I should be feeling unwell.

24 hour Urine

T3 292pmol/24h (Range 800-2500)

T4 905pmol/24h (Range 550-3160)

T3:T4 Ratio 0.32 (Range 0.50-2.00)

If Chiquititas is desperately low at 445, I am unsure how to classify my result at 292 .

Kind regards



Perfect information about the difference between blood and urine testing, thanks for posting :-)

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Hello C would you mind to send me details of the private Dr and who you used for you 24 Urine test thanks very much


Which doctor on the uk did you see? I note your post is 4 years old, has this doctor helped you?


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