Ft3/Ft4 calculator

Hi there,

As you know doctors in UK don't really care about t3 and t4 often saying it's in range, but you wondering why you still feel like..

Here's a calculator that will show you % levels of t3 and t4 in your body, all you need is your results and lab range used for test (which can be printed out by your gp). Calculator is in polish but you will all get it, first the result and then the range, then click on 'oblicz' and you will see your hormones in % :)


67 Replies

Very useful. You can add the two results together to get a total percentage. The average total for the presumed healthy population is 100%. So you can get a measure which is not reliant on TSH (which can be influenced by a number of factors).

This isn't accurate. You cannot add together the percentages of two hormones with very different activities. T4 is largely inactive and T3 active. So patients with relatively more T4 and less T3 could give the same numerical result as those vice versa with relatively less T4 and more T3. But their thyroid status wouldn't be the same. You could easily mix up hypo and euthyroid subjects according to their particular circumstances. It's like adding apples and pears and getting a celery stick.

I've found it useful. I used a more complex equation of 4 x fT3 + fT4 to get a factor for total hormone levels (as fT3 is four or five times as potent as fT4, as measured by effect on TSH). Adding up the two percentage figures is not as accurate but far simpler than complex calculations using mid-points of reference intervals.

T4 is available to tissues and deiodinased within the cells. Some of the T3 so produced is realeased into the blood (anyone know what percentage?). So, both T3 and T4 are used by the cells, at least by those cells that have deiodinase capability - which includes the heart and brain. For healthy subjects as fT4 rises TSH falls and deiodinase activity reduces with fT3 tending to stay level or fall (whilst they remain within the reference intervals).

I needed high dose L-T3 treatment for a long time and this has led to my hypothalamic pituitary thyriod axis being down-regulated. I am now OK on moderate doses of L-T3 and a little L-T4. A year or two ago I carried out an experiment over a period of six months or so. I came off medication and calculated my 'index' at around 82%. I gradually added L-T4 and as I did my fT3 decreased, until I was on 125 mcg L-T4 when my fT4 went outside its reference interval. Then fT3 started to increase, I assume due to type-1 deiodinase taking over. On 100 mcg L-T4 my 'index' was 89%. I am fortunate in that I have some results from before I started medication, when my axis was fine. At that stage my 'index' was 98%.

From a practical point of view I was symptomatic when my 'index' dropped to 83%, I was OK when it was at 89% although I now need some L-T3 to overcome some cognitive problems.

I believe this simple test is an excellent way of spotting a down-regulated axis, which can occur after a period of hyperthyroidism, either by taking large doses of hormone or due to a period of overactivity in autoimmune disease which may not be spotted. So many GPs and endocrinologsts say patients are OK when both fT3 and fT4 are within interval but perhaps both in the lower half or quartiles. We see lots of patients posting on this forum with 'normal' thyroid function tests but lowish fT3 and fT4 and average TSH.

(Diogenes, I can send you a copy of my data if you like. I don't want it in the public domain as I might seek to publish it in a paper).

I still hold my ground. The FT4/FT3 ratio is a far better way of assessing a) absolute levels of hormones compared to the reference range and b) conversion of FT4 to FT3. If in a patient on T4 only you get a ratio greater than 5 then poor conversion is likely. On T4/T3 combo try to get around 3-4/1 and on T3 only, keep FT3 up in the range - the ratio no longer applies because you have negligible T4.

So, fT3 made intracellularly then leaks out into the circulation from cells other than the thyroid?

Yes that's true: the thyroid is only a minor direct supplier.

All I know is I function best with high fT4 because fT3 never gets higher than 50%. But with taking T3, it's not good at all. Getting fT3 in the mid 5s (on average) was a disaster for blood glucose. Putting levo and liothyronine dosages so that both levels were like a normal healthy human did not make me into a normal healthy human.

Two types of deiodinase.

Type-1 mainly from the liver and kidneys which produces equal amounts of T3 and rT3 is promoted by high hormone levels, especially fT4.

Type-2 which produced in other tissues including the brain and the heart occurs closer to the cell neucleus and only produces T3. During cold exposure there is a great deal of type-2 deiodinase in brown fat to generate heat (up to 50x as much). However, this T3 doesn't find its way into the blood, which is just as well.

So I'm not sure anyone knows how much T3 leaves the cells and enters the blood. It probably varies tissue by tissue. All T3 (other than T3 from the thyroid) is produced from T4 to T3 conversion in peripheral cells, but if you measure T3 you don't know how much has come from type-1 or type-2 deiodinase.

Do these people feel well or do they just live long?

fT3 around 4. fT4 around 15. Meantime supposedly fT3 ideal is over 5.0.

J Gerontol A Biol Sci Med Sci. 2010 Apr;65(4):365-8. doi: 10.1093/gerona/glp200. Epub 2009 Dec 16.

Low serum free triiodothyronine levels mark familial longevity: the Leiden Longevity Study.

Rozing MP1, Westendorp RG, de Craen AJ, Frölich M, Heijmans BT, Beekman M, Wijsman C, Mooijaart SP, Blauw GJ, Slagboom PE, van Heemst D; Leiden Longevity Study (LLS) Group.

Author information



The hypothalamo-pituitary-thyroid axis has been widely implicated in modulating the aging process. Life extension effects associated with low thyroid hormone levels have been reported in multiple animal models. In human populations, an association was observed between low thyroid function and longevity at old age, but the beneficial effects of low thyroid hormone metabolism at middle age remain elusive.


We have compared serum thyroid hormone function parameters in a group of middle-aged offspring of long-living nonagenarian siblings and a control group of their partners, all participants of the Leiden Longevity Study.


When compared with their partners, the group of offspring of nonagenarian siblings showed a trend toward higher serum thyrotropin levels (1.65 vs157 mU/L, p = .11) in conjunction with lower free thyroxine levels (15.0 vs 15.2 pmol/L, p = .045) and lower free triiodothyronine levels (4.08 vs 4.14 pmol/L, p = .024).


Compared with their partners, the group of offspring of nonagenarian siblings show a lower thyroidal sensitivity to thyrotropin. These findings suggest that the favorable role of low thyroid hormone metabolism on health and longevity in model organism is applicable to humans as well.

Comment in

Thyrotropin serum values and 3-year mortality in nonagenarians. [J Gerontol A Biol Sci Med Sci. 2010]

I agree with you diogenes, a ratio of FT4/FT3 makes a lot more sense since it gives an idea of how much FT3 is getting generated from the FT4 stored in the blood.

In the US they do TT3/TT4 (i.e. the other way up (and total)) and the answer should be between 17-20 BUT ….no-one has an intuitive feel for this ratio because they use different units for each measurement so it is not strictly a ratio. It is a "ratio" not used very often because most doctors cannot handle this. The measurements, just in case you ever run into this are: ng/dL for TT3 over mcg/dL for the TT4, these are used because these are the units in which TT3 and TT4 are measured - sounds a bit Ad Hoc to me.

In any case, adding the apples to oranges sounds silly.

This is great, just need to pay and get the ft3 done! Why oh why can't our doctors use this?

Wow! Just done mine and it comes to 30.19% total. My Diabetes consultant/Endo says this is normal so I should stop worrying about my thyroid and just concentrate on having CFS/ME.

What planet are they on? :0

Great link ziel. Thank you x

A useful tool to see where you are in the range - are you in the top or middle, etc etc. But the idea that added both together allows you to see where you are cf 'the supposedly healthy population' who would score 100% is absolutely not the case. For a number of reasons and not just the maths.

Thanks to this calc I found out my t4 was about 70% and t3 only 20%, both still in range but I was walking dead and doctors of course couldn't help me... Since then I'm on NDT and I'm alive wooooo!

My understanding is that the ideal is a T4 in the top quarter, and T3 in the top third (assuming you are taking T4 and hoping it gets converted, if on T3 substantially or only it doesn't matter). These translate to 75%+ for fT4, and 66%+ for fT3.

So the above comments about adding them together and getting 100%, would imply having both of them around 50%. Many people would be slightly hypo with these numbers I think?

Actually, the T3 should be higher than the T4 in their respective ranges, if conversion is good.

That is true with healthy adults. The Turkish study put fT4 around 13 - 14 (range up to low 20s) and fT3 in the mid 5s (range up to 6.5). TSH average 1.4.

So, what was the bottom of the ranges?

I think it was the usual, 9 for fT4 ad 3.8 for fT3. I have to find the paper again if you want me to link to it. The study was done on normal young adults and blood was taken hourly over a 24 hour period. That's the paper which shows the diurnal curves for TSH, fT4, and fT3.

Ok, so both about mid-range. I don't need to see the paper, thank you. :)

But the question always arises - in my mind, anyway - how did they know these people were perfectly healthy and didn't have any sort of thyroid problem?

The study group were all probably university students, young adults.

But what about the long lived cohort in the Leiden Longevity Study? If low fT3s = long life, then what? Do these people feel good? Reminds me of the American Endocrinologists who have a position about post menopausal women who become mildly hypothyroid: do not treat because they live longer if a bit hypo. Arghhh. So do post menopausal women who feel like crap get referred to endocrinologists who test, find them mildly hypo and send them on their way? Or what? So damn annoying.

Who are they to judge, anyway! A short life but a merry one, I say. I don't want to linger on for years in misery. People should be informed and given the choice!

They make more money if you linger.

Yes, I'd forgotten that. Well, they won't make much out of me! No matter how long I linger, I will not take their horrible drugs! I won't even go to see them. So there! lol

Definitely best. Just make sure your Will is in order and your beloved relatives know where to find all relevent paperwork. ;)

Already done! lol

Whats the point of living longer if you are like a zombie?

That's why you need to self medicate and take care of yourself, coz doctors obviously don't give a damn lol.

50% for each is where the average person is. So it the only problem is a failed thyroid gland treated patients will, on average, be around half way on each scale. However, in order to get fT3 around 50% it is known that patients on levothyroxine only need their fT4 higher, around 70% or 80%.

T4 in the top quarter, and T3 in the top third is not where most patients with primary hypothyroidism should be. They would be hyperthyroid. However, most people on these forums have more complex conditions with perhaps some peripheral resistance to thyroid hormone. Perhaps many of this cohort do well with their hormone at these levels. However, this should not be seen as a target.

No. The target should be where you feel well. You can get too hung up with numbers!

Yeah, don't add them, they are separate numbers.

I like that it tells you where you are in range, what i would really like is a calculator where you can put in what dose you are taking and input the results and if they are not optimal it will tell you what dose to take to make you optimal...oh...i think we have one of those already...an endocrinologist!! lol!!!

Joking aside i did do the calculation, bearing in mind i take T3 (30mcg per day) and T4 (150mcg per day) mine has come out as T4, 46.28% and T3 26.67%, i dont see any relevance or science behind adding the 2 together, i am not well on this combination, i would be interested to know what the ration of t4/t3 is in NDT as we are reading some very positive stores in particular Ziel who seems to have found the right answer for her, only problem for me is i'm vegetarian!!

If you are on t3 dose but your results still showing low % it looks like you are on too low dose. 150 levo is quite high so maybe this is the problem with digestion? Do you take your pills on empty stomach at least 1 hr before first meal?

Hi Ziel, yes i always take pills on empty stomach at least 1 hour (more like 4 or 5) away from food. Im thinking perhaps the ratio of levo/lio is wrong for me though, do you know what it is in the NDT you are taking?

Sounds like you aren't converting much of the T4 you're taking, and your dose of T3 isn't high enough. You don't need to calculate percentages to know that. The numbers leap out and bite you on the nose.


Thanks for the reply but what do i increase....levo or lio? Presume Lio seeing as the levo isnt converting and if so guessing i reduce the levo and increase lio or not to bother seeing as it doesnt seem to be doing much?

You need to increase levo, your body should convert some of it to t3, if it doesnt help then increase lio as well

Oh, but the unconverted T4 sloshing around can cause a lot of problems! It can convert to rT3 and block the T3 receptors.

Much better to reduce the levo and increase the lio, if you ask me!

I am wondering GG if this is what happened to me before taking T3 when i was on 200mcg lio my FT3 was rock bottom but my FT4 was slightly over the top end of the range which i guess is what highlighted a conversion issue? Could do with some kindly advice of what combination to try? im thinking125 levo and 40mcg lio?

You mean 200 levo? Yes, a conversion problem.

Impossible to tell you what dose you should be on, but myself, I think 125 levo is a little high for someone who converts as badly as you do.

You're taking 30 mcg T3 at the moment? Increase slowly, 5 mcg at a time. You have no idea how much is going to be the right dose, because you've had so little T3 in your system up til now. But, every time you increase your T3 by 5 mcg, reduce your T4 by 25 mcg. Do it slowly and see how you feel.

It's possible, you might end up needing T3 only.

Whoops GG! Yes defo levo 200mcg! Okay...so...until now i have been upping lio every 6 to 8 weeks by 10mcg until my doc has said no more until you see Endo...how long should i leave between dose increases on T3? i know T4 is about 56 - 8 weeks but lio i thought was much quicker? I still dont get why i need to take levo and lio if the levo is not converting very well? i realise you have said it may be that i could end up taking t3 only but how will i know this? I thought i knew what i was doing but i was wrong by adding a little t3 to t4 but maybe this is the wrong recipe for me afterall!

You have to go by how you feel. But it's not a good idea to just dump the levo all in one go, reduce slowly, it's kinder to your body. It is possible that T4 does have it's role to play in the body, even if you can't convert it very well. So, you need to see how you feel after each reduction. Do new symptoms appear?

I think, at this stage, it would be advisable to increase the T3 by just 5 mcg every 4 - 6 weeks. You're probably getting near optimal, and you don't want to over-shoot it - which is easily done. Listen to your body in between increases, to know how you feel.

Thanks GG. I know its hard to say for sure and ive read low and slow so thats what i'll do but i wont be able to get another blood test until i see the endo so i will have to go by how i feel until then.

Yes, learn to read your body, that's the clue.

Lc1973, those numbers mean you are underdosed. The T4 you're taking isn't doing you much good, as the fT3 is so low! (T4 must get converted into T3 to be used).

This calculator isn't doing anything fancy, it's just accurately working out what you can eyeball by looking at the blood test (or work out on your fingers like I do if you are less numerically minded). Your numbers need to be high in the ranges to make you feel well.

Thanks Silveravocado. Ive just replied to Greygooses similar message, Is there any point then in me taking levo if its not converting into the active T3 do you think?

But you need both, t3 and t4.

Hi Ziel. Buy why do you need both? Surely folk who dont have a thyroid dont produce any t4 do they? Or is T4 produced in other organs as well as thyroid? im getting confused i think!

That's why you need to supply it from outside. Both hormones are needed in your body to control metabolism. In you have thyroid it might produce too small amount of t4 hence why you need tablets. If you don't have thyroid you need to source it as well and keep it up til end of your life. Your body will convert some of t4 to t3, but it might not convert properly so you need additional dose of t3 along with t4. Both hormones should be about 70%.

Crikey...ive a long way to go in that case Ziel! My doctor wont give me any more T3 and has referred me to an endo but my appointment is not until November!! I have some self sourced T3 (tiromel) which i could add to my regime but i will need to split the dose as i seem to be sweating a bit post dog walkies, and or i could add some more levo as i have some spares from previous dose changing!!

Add 25 mg of levo and see what happens

Lc1973, I would do as Greygoose suggests, very slowly increase the T3 and drop the T4. It's not true that your body needs T4, it's just the storage version of the hormone, and is converted into T3 by your body to use. Some people are bad at the conversion, so need to take T3-only.

The only thing I'd add to GG's advice is to get a private blood test every 6 weeks to be doubly sure. That's what I do, as I'm in a similar boat adjusting every 6 weeks. I get mine from Blue Horizon. It cost well under £100 to get the TSH, fT4 and fT3. Or a tiny bit more to get all the relevant vitamins, too.

Thanks SA. I'll have a look at Blue Horizons website. Hope you are getting somewhere with getting the right balance in dose!! I'm 6 years in and still struggling but i do think the T3 is helping a little.

Thanks for this Ziel, i was looking around on the internet yesterday trying to find a reason to take T4. I shall read in more detail later.

The ratio of T4:T3 in USA desiccated thyroid is just over 4.2:1.


This is simply the ratio by weight.

If you took, say, 100 micrograms of levothyroxine and converted it completely to liothyronine, you would get just under 84 micrograms of liothyronine. Each molecule of T3 is that much lighter than each molecule of T4. No-one ever takes that into account.

My FT4 is 75% and my FT3 is 2.5%. O dear. No wonder I feel like doo-doos, if this is anything to go by.

Hi just calculated my numbers. I am not on any thyroid medication as I am deemed to be well! What should I consider doing judging by results

FT4 11.43 The standard of 12 down 22

FT3 6.02 The standard of 3.1down 6.08


FT4 -5.70% [the result of 11.43, standard (12 - 22)]

FT3 97.99% [profit 6.02 standard (3.1 - 6.08)]

All suggestions welcome.☺

This is a very unusual result if you're on no medication at all. I suggest starting your own thread to ask about this, as more members will see it and comment.

V interesting ziel.

FT4 5.88%

FT3 53.33%

Any thoughts?

I wish I knew half of what you all are talking about cause I could really use the k nowlege since the disorder has taken over my entire family. We have all been I'll for years

Oh 7658, that brought tears to my eyes!

It's not easy when you are just starting out. You have to read everything you can. Start with the simple stuff to begin with, get very basic explanations off the internet, then get a book like STTM (which stands for Stop the Thyroid Madness - yes, a stupid title, but its a good book) or Tired Thyroid by Barbara Loughlin(sp). I studied like a maniac for about a year, yes, I obsessed because I was so scared of what was happening to me and what little help I was getting from docs. Don't worry if this study takes up most of your spare time for a while (could be a year!) it will be worth it. Keep asking questions here, even simple ones. You have found us, that is a good start. This thread was a bit advanced, don't let it discourage you.

Thank you so much for your words of encouragement. I really appreciate the time everyone takes for each other on this site.

Due to my apparent euthyroid status (results *always* in range despite symptoms), the medical profession considers me to be 'normal'. The percentages according to this calculator suggest otherwise, if the values should be over 50%:

FT4: 35.9%

FT3: 28.92%

Or have I misunderstood?

They are still in range for a gp, bet they don't even know about calculators, maybe they would look at us differently, obvious low hormone level here.. What dose are you on?

I'm considered euthyroid, so have never been offered medication (I have a diagnosis - since 1989 - of ME). My last NHS TSH result was 3.84 (0.27-4.2) but dismissed by the GP as it was in range.

I also have low (but just in range) folate level: 10.9 (10.4-42.4) but good B12 levels: 621 (over the insufficient level of 250).

I seem to fall through all the nets!

Please help me to understand.

My result was

FT4 1.26 (range 0.97 - 1.72) calculated % - 38.67%

FT3 2.8 (range 2.1 - 4.2) calculated % - 33.33%

Doctor said to me that my result is in normal range, and he could not help with my symptoms.

Shall I consider about my thyroid? or is it a normal as doctor said?

Thank you

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