Help with normal blood ranges when taking T3

Hello there,

I have not posted or responded for quite some time. About a year ago I switched to T3 and my doctor is concerned that my thyroid function blood test are all over the place. Can anyone help me to work out which are the best blood tests to have a) toshow that my T3 levels are good and b) that am not hypothyorid?

It is with great regret that I have not been able to engage with the forum but I have become very ill. I have developed sever aortic stenosis and have aso found my vitamin D levels are low again and I have to go in to hospital tomorrow for a blood transfusion because my haemoglobin levels are also low again!! I should then have by pass surgery to replace the valve. Hopefully, I will then be much better.

My GP has now offered to help me manage T3 but he does not seem to understand the blood test results. I am too ill to go and get a printout of blood results but he said total T3 was double what it should be. I am not sure about FT3. He also said THS was suppressed but then shouldn't it be? If anyone who his taking T3 can post healthy blood results that would be great. I know we are all individual but it would help..

I would be really grateful for any help before I am admitted (or should that be committed).



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12 Replies

  • As far as blood test numbers go TSH is the best indicator with a value of around 1.0 to 2.0. T3 figures are not so useful and in any event you should not take any form of T3 just before the blood is drawn, leave at least 8 hours.

    However, if someone is having to take T3 only, or mainly T3 they are not suffering from a case of simple primary (thyroid gland failure) or secondary (pituitary) hypothyroidism and so medicating by blood test results only is of no use. Obviously if the TSH is suppressed, or the fT3 above the upper reference interval limit there should be caution but the prime objective should be to monitor the patient's signs and symptoms. They need to adjust your medication according to your response to it rather than the blood test numbers.


  • To say that "TSH is the best indicator with a value of around 1.0 to 2.0" is extremely misleading and incorrect. There are people on here who would be overdosed with a TSH less than 2.9 and I am an example of someone who was severely underdosed with a TSH of 1.4, or even 1.0.

    By suggesting there is an ideal TSH figure is playing into the hands of the people who set more store by blood results rather than by clinical signs.

    T3 figures are actually extremely useful if a person is taking Levothyroxine because it can pinpoint any conversion issues, so long as the free T3 is the one which is measured. This is because some of the 'total T3' will be bound to protein and thus unusable.

    I agree completely, though, with your comments on T3-only, and medicating according to response. I would, however, like to add that in some cases of tissue resistance even the upper limit of the T3 range may well not be relevant.

    When taking T3 it is perfectly OK for the TSH to be very low so long as the T3 is not too high (Dr Toft, 'Understanding Thyroid Disorders'), and recent research has suggested that it is not a suppressed TSH which can cause problems, but an elevated T3. So, really the Free T3 is the figure to watch. And, of course, how you feel.

    Marie XX

  • All very interesting but I do not think my doctor will understand. I suppose what I requested was what is the best test for T3 and what will help demonstrate that I am well on T3. Please see next post.

    Thanks for help.

  • Sorry but I cannot disambiguate the message. Too complicated for me but thanks any way.

  • "As far as blood test numbers go ............... TSH is the best indicator with a value of around 1.0 to 2.0." The caveat "As far as blood test numbers go" restricts the comment to the interpretation of blood test results. T3 (I should have written fT3) assays are of little use if the patient is taking T3 containing medications as the T3 is unbound (free) until it has bound to the serum transport proteins. The fT3 figure simply represents the T3 the patient has recently swallowed and is not a fair representation.

    In the case of levothyroxine the fT3 figures are reliable (as are the fT4) because of the long half-life, most of the thyroid hormones has been in the blood for sufficient time, the fT3 has come from the levothyroxine and so the figure is accurate.

    fT3 figures can give an indication of deiodinase (T4 to T3 convesion) but it's very difficult to interpret the numbers. As fT4 increases deiodinase activity will decrease in order to maintain cellular fT3 levels (and vice-versa). So as fT4 increases fT3 will increase to a lesser extent making interpretation of the numbers difficult. Deiodinase is also stimulated to some extent by TSH, which makes sense as a failing thyroid can be supported by increased deiodinase activity.

    The pituitary is fully dependent upon type-2 deiodinase, so if there is a conversion failure TSH will be higher. Unfortunately in this case TSH is usually only double it's normal level. So if it was 1.0 it will rise to 2.0 - not much help. Thus the fT3 figure can help suggest a conversion problem (TSH is more helpful) but it can only give an indication. This problem is confounded by the possiblity the patient may have primary or secondary hypothyroidism thus skewing the numbers. A further complication is that there is still some debate about whether and to what extent T3 produced by type 2 (intracellular) deiodinase finds it's way into the blood. I think it does but the evidence is not 100%.

    Tony Toft's suggestion that it's OK for patients taking levothyroxine to have high fT4 levels (and hence low TSH) if their fT3 is within the reference interval is I think based more on gut instinct rather than science. I'm not arguing against patients having a suppressed TSH (I did) but making an academic point. In the case of thyroid hormone resistance it's no use trying to keep the patient within reference intervals, sings and symptoms are the only way to go.

    Lastly, if the patient requires more than about 10 mcg L-T3 (from liothyronine or Armour) then it's likely their problem is not due to a conversion failure unless thier fT3 level is very low or they have selenium deficiency. (I'm assuming they have not been hypothyroid from birth). That's not saying they don't need T3, just that it's due to some other unknown thyroid hormone utilisation problem.


  • Hi Jim,

    Thank you for scientific explanation. Too much information for this very ill person.

    Can you tell me best test to demonstrate T3 working as well as signs and symptoms.



  • Sorry. When your doctor said T3 was double .... he was probably referring to fT3. If we assume you take your T3 twice a day then you should hold off your morning dose until the blood has been taken. Purely from a blood test point of view and assuming there is no hormone resistance I would expect your TSH to not be suppressed. It would be OK for your fT3 to be a little above the upper limit of the reference interval (up to 9.0?) because you have no T4. T4 enters the cells where it is converted to T3, so T4 in the blood contributes to thyroid hormone action.

    Having said the above if you are having to take T3 only then you do not have a simple thyroid gland problem. Consequently you cannot rely on the blood test numbers, you have to go by signs and symptoms. This is hard for doctors to accept but the patients condition and their response to thyroid hormone is the reality, the thyroid theory has to take second place.


  • 'Having said the above if you are having to take T3 only then you do not have a simple thyroid gland problem'.

    Could you please elaborate on this? Are you saying that people who have to take T3 only have other problems besides the thyroid issue or that their thyroid problem is more complex or either or both? I'm asking as I was told i didn't have conversion problems and yet I couldn't tolerate T4.

  • Jow11,

    It's possible that some people may be intolerant to T4 tablets and not to T3 because of an intolerance to one of the fillers, although I'd guess this is unlikely as they tend to have similar makeup. If it's not down to the tablet formulation then logically you should be able to tolerate normal levels of T4 as it's the same molecule as your thyroid produces. (Not strictly correct as the tablets are levothyroxine sodium).

    Many patients require liothyronine (L-T3) rather than levothyroxine (L-T4) but I don't believe they have a conversion problem, there is very little evidence anyone has substantial conversion issues (it's a deep subject so I will not go into it here). Without doubt they require some form of T3. Patients with certain polymorphisms of the DIO2 gene do need some T3, perhaps 10 mcg of liothyronine but they do not have major conversion issues, they do OK until they become hypothyroid.

    My view is that patients with thyroid hormone resistance, who require high dose medication will need a form of T3 rather than T4. This is because they need hormone levels well above the normal. However the deiodinase proteins regulate T3 in the cells by reducing T4 to T3 conversion and speeding up T3 to T2 conversion in the presence of high levels of T4. This is not a conversion problem, it's how the body is designed to work. However, because of the hormone resistance we need to overcome this mechanism and by taking T3 we are able to bypass part of the cellular homeostasis. This is especially important in the brain which keeps T3 levels within a narrow range. This is my theory, but it explains why patients who clearly need high doses do well on T3 and not on T4, especially in terms of cognitive function.

    There are likely to be a number of other reasons why people need T3 which we haven't discovered yet. Most people who need T3 do not have a conversion problem but the clinical response proves they do need T3.


  • Hi Jim, thank you very much for your comprehensive reply. I know that in my case it wasn't a problem with the filler as I tried compounded T4 and still couldn't tolerate it.

    My dr did mention the possibility of thyroid hormone resistance but would 90 mcg (this is what I take) of T3 be regarded as a high dose? I used to think so but then I heard of people on this site taking 180 mcg or more.

    Thanks again.

  • Thanks Jim. this makes more sense to me at the moment. I have had Hashimoto's Thyroiditis which developed into Myxodaema which is why it is fine to take T3. It is a while since I read data on blood tests for those taking T3 only. I used to go to a private clinic but cannot afford it any more. I had a chat with one of the GPs in my surgery and he said he would help monitor T3 levels. However, he performed function tests for thyroxine and as you would expect results were all over the place. He has asked me to ask people on the Thyroid UK forum for advice because he wants to know more. Most unusual but fantastic for me. Anyway I feel great on 50mcgs of triodothyronine and 25mcgs of thyroxine. So signs and symptoms are good. Off for transfusion and finally a proper investigation as to the type of anaemia I have.



  • I would keep it simple. The blood tests are of little use for patients taking mainly T3. T3 is about 4x as potent as T4 in terms of blood levels. However, in tablet form T3 is about 3x as potent, allowing for the difference in absorption and half-lives of the two medications. So you are on the equivalent of about 175 mcg thyroxine.

    There are a number of accredited websites that generally recommend a dose of 40 - 60 mcg liothyronine for primary hypothyroidism, this is one from the BNF " " . (Note that the BNF states 20-25 liothyronine is equivalent to 100 mcg levothyroxine, this is incorrect, it relies on out of date research which doesn't take into account the pharmocokinetics of the tablets).

    So your dose is pretty normal for someone whose thyroid gland has failed. The blood tests are not much use so it's important to monitor your signs and symptoms. When you can it's better to take a little more thyroxine and less liothyronine but only if it doesn't make you worse. I'd recommend you buy a blood pressure machine (they're cheap) and monitor your pulse weekly, if it should go up for a week or so then you may need to reassess your medication. Beware that your pulse will go up if you get a cold or 'flu whilst your lungs are a bit bunged up.


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