Thyroid UK
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How to calculate optimal levels?

You often read that FT4 and FT3 should be in the upper third or 50% in range. How do you calculate where your level is right now?

I understand it's different for everybody, what a good level is for me could be hyper for someone else.

But like FT3, some say it has to be in the upper 75%. When I calculate that with the ranges of my lab someone said that is way too high and would be considered hyper.

So, how do you calculate at which percentage your results are now and what is the goal?

52 Replies

Flower, post your FT3 and FT4 results with the ranges and I'll show you how to calculate where you are in range.


Thank you! My FT4 2 weeks ago was 17,8 (range 10-19).

FT3 before starting treatment was 4,4 (range 3,5-5,9).

I will get FT3 tested again in 3 months and want to know what is considered optimal. I read somewhere it should be at 75% so that makes 5,3. Is that too high?

I know you shouldn't focus too much on the numbers but if I know what is optimal I know if things are going okay and if not I could ask for T3.


Flower, I've reposted to correct an error in my figures. I've deleted the previous post.

For FT4 subtract the lower range figure from the top figure. Multiply that figure by 0.75 and add to the lower range figure subtracted ie

19 -10 = 9

9 x 0.75 = 6.75

10 + 6.75 = 16.75 (the bottom of the top 75% of range).

So, your FT4 17.8 is nicely within the 'ideal' top 75% of range.

FT3, as above but multiply by 0.5 as your FT3 is just below half way in range ie

5.9 - 3.5 = 2.4

2.4 x 0.50 = 1.2

3.5 + 10 = 4.7


I would also love to find out about my levels, please. When I was on 5 grains of Armour, my levels were as follows:

TSH >0.01

FT4 1.18 (ref 0.7-1.5)

FT3 2.9 (ref 1.8-3.8)

My doctor thought I was undermedicated, and I wanted me to raise Armour until my FT3 levels reached at least 3.5. However, I always go to the lab 24 h after taking my labs show what my FTs looked like 24 h after taking Armour. In your opnion, do my levels look OK, or lowish, considering that I went to the labs 24 hours after taking any NDT...?

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Anna, I made an error in my rely to you which I have now deleted and repost below. Bottom range value should be subtracted from top of range, multiplied by % value and added to bottom range value.

1.5 - 0.7 = 0.8

0.8 x 0.50 = 0.4

0.4 + 0.8 = 1.2 corrected figs 0.4 +0.7 =1.1

So, your FT4 1.18 was a gnat's below half way through range but T3 in NDT can lower FT4 so being good in range isn't important.

3.8 - 1.8 = 2.0

2.0 x 0.5 = 1.0

1.0 + 2.0 = 3.0 corrected figs 1.0 + 1.8 = 2.8

so, FT3 2.9 is just above half way in range. As you left 24 hours between last dose and blood draw you can probably extrapolate your FT3 2.9 + 20% to get an estimated reading of 3.48 as a truer reading of your normal circulating hormone and near the top of range. I don't believe FT4 being a storage hormone is extrapolated in the same way.


Would also like to know what you think. Taking T3 only. TSH=0.01. FT4= 3.2. FT3=4. RANGE FROM LAB= FT3= 3.8-6.0 FT4= 8.4-19.1 TSH=0.3-4.8. Thank you.


Erfathyroid, I made an error in my rely to you which I have now deleted and repost below. Bottom range value should be subtracted from top of range, multiplied by % value and added to bottom range value.

Your FT3 4.0 is good, in the top 75% of range.

6.0- 3.8 = 2.2

2.2 x 0.75 = 1.65

1.65 + 2.2 = 3.85 corrected figs 1.65 + 3.8 = 5.45

FT4 is below range because you are taking T3 only.


Thank you so much. If someone is in the optimal FT3 range, can you still benefit from adding T3?


Flower, Levothyroxine should have improved your FT3 since November but it can take a few weeks for symptoms to resolve after bloods are good. It took 8 weeks before I felt anywhere near as good as my bloods indicated I should be feeling. I advise checking your FT3 level before considering adding T3 or switching to T3.

I had low T3 which improved with the addition of T3 to Levothyroxine but more important to me was T3 seemed to calm the adverse effects which built up when I was on T4 only.


Thank you Clutter, I will have a blood test in may. I'm just 2 weeks on the optimal dose so I will wait and see. I'm just afraid that it won't get better so that's why I'm already looking for a plan B.

Taking supplements and levo so I guess I will have to give my body time to process it.

Will have ferritin tested this week so will see if that is optimal.

Thank you, wouldn't know what to do without this board.


Flower, don't worry about plan B yet. Plan A just needs more time, it is a very slow process *sigh* and you are doing the right thing in looking to optimise your vits and mins. I hope you'll be feeling much better in a couple of months.


I hope you don't mind me adding to the discussion here, but this doesn't seem to work with my TF4 result:

13.6 (9-22)

22-9 = 13

13 x 0.75 = 9.75

9.75 + 13 = 22.75

Something is definitely going wrong here and try as I might I can't figure it out!


Because you're adding the whole range (13) to the 75%, not the bottom of the range value (9). For a result in the top quartile of the range you need to be at 18.75 ((13x0.75)+9).


Of course! Thank you. Ever since I was diagnosed with ME in 1989, numbers have been a huge problem - you wouldn't believe that I had taken A level maths!

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You were right to flag this up, some of my calculations are really not making sense either.


Sorrel, my error. Bottom range should be subtracted from top range, subtotal multiplied by % value and added to bottom range. I've corrected the errors and advised posters.


Thanks Clutter, that is what I had thought, I had pencil and paper out last night the calculation should be :

1. Subtract the bottom of the range from the top.

2. Multiply that answer by the percentage (expressed as a decimal no.) say 75%=0.75

3. Add the bottom number of the range to the second answer

Thanks again


Sorrel, if only I had expressed it as elegantly, thank you.

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My immediate response to you posting is below this one... but I've just looked back at the formula above and it says to add to the number just subtracted (9 in the example). To apply your formula (which is the correct one), it should be 10 in the example? Or have I really lost it?


I think Clutter may have calculated it wrongly. You should add the calculation of 75% of the entire range to the lowest range value to get the figure for the upper quartile.


You can try a calculation to show the uppermost value by taking the range (in your case 13), multiplying to find 100% (i.e. multiplying by 1) and adding it both to the lowest range value (i.e. 9, so 9+13=22, which is the top of your range) or to the range itself (i.e. 13+13=26, so over the top of the range). Your maths A-level will come back to you eventually, I'm sure! But finding the extremes of values will always test the validity of a formula. (I'm fresh off an Open University maths course.)


That was my calculation too...


Hose, you are quite right. Bottom range should be subtracted from top range, subtotal multiplied by % value and added to bottom range. I've corrected the errors and advised posters.


Yes that was my way of working it out too- we have 2 equations working here on this thread. One that adds the bottom range and the other that subtracts the reference ranges and adds that number to the total. This is confusing...


Hose, Ann_g_k was calculating 75% of the range correctly. Her FT4 is low in range, nowhere near the top 75% of range.


Many thanks Clutter. The FT3 is just as bad:

FT3 is 3.14 (2.63-5.70)


5.7 – 2.63 = 3.07 x 0.66 = 2.02

2.02 + 2.63 =4.65 (top 1/3 of range)

Is this correct?


Ann_g_k, I deleted my previous response because there was an error in the formula. Subtract the bottom range from the top, multiply the value % and then add the bottom range ie

22 - 9 = 13

13 x 0.33 = 4.29

9 + 4.29 = 13.29 so your FT4 is slightly over 33.3% through range.

You've calculated 75% of FT3 range correctly. Your actual FT3 percentage of range is very low.

5.7- 2.63 = 3.07

3.07 x 0.17 = 0.52

2.63 + 0.52 = 3.15


Typo alert... 33.3%, not 333%


Thank you, hawkeye :-D I'm having problems with nails too long and sticky keys but still...I'm not doing too well on this thread :o


You are, I understand it now so it is very helpfull!


Thank you, Flower :)


Well, I've never been diagnosed as hypo - usual problem, all results within range (although my endo doesn't know about the FT3 yet). Latest TSH is 2.53 (0.35-6.00), down from 3.36 in May 2014. I see an endo at a local ME clinic and I'm waiting the synacthen test as he thinks I may have adrenal insufficiency.

Then again, he thinks the FT4 is fine... and also Vit D, which at the height of summer was 50, the lowest value in the range. He didn't make the mental leap that if it was so low in summer, what was it like in the winter. Having supplemented ever since, it's now 124.

Needless to say I don't have much confidence in the endo.


Ann,It's hard to say what optimal thyroid results are for individuals unless they know what thyroid levels were before becoming unwell. I think the suggestions for top 75% of range for T4 and the top third for T3 are geared towards patients on thyroid replacement.

You'll know that TSH >2.5 and low FT3 and FT4 are problems for some people who do experience significant symptoms despite being within range.

Adrenal insufficiency can make it difficult to tolerate thyroid replacement so its good your endo is investigating.


Hi Clutter can you tell me why in the first (flower) equation you multiplied by 0.75(FT4) and by 0.66 (FT3)and in the second equation (Anna)you multiplied it by 0.50(FT4), and 0.5 for I missing something, thank you


Dépends if you want to know the top quarter, top third or half-way through the range.

If you're anything like me, brain-fogged, multiplication isn't easy. I do it this way...

Take flower's range of 3,5-5,9 for T3 :

5.9 - 3.5 = 1.4 divided by 2 (for mid-range) = 0.7 + 3.5 = 4.2

So mid-range is 4.2. Flower's T3 was 4.4, so just slightly above mid-range. Which would be too low for most people.

But my definition of 'optimal' has nothing to do with a lab range. It's when your hypo symptoms have all gone. Provided, of course, that you've addressed all your nutritional deficiencies.

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Calculating is also hard for me, my brain can't do it. You say just above mid-range is too low for most people, what range is the best you think?


I Don't think in ranges. I haven't had a blood test for nearly a year. I go by symptoms. Right now my pulse is down to 64, and i'm thinking I need an increase!

From what I've read on forums, most people feel best with their FT3 up near the top of the range - or even slightly over. Don't forget we've probably been hypo for a long time before getting diagnosed, so our bodies no-longer work like so-called 'normal' people. It's perfectly possible that we need higher than average levels of T3 to function. But the best level - the right level - is the one at which you feel well.

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That's why it takes so long..I rather had a broken leg, much easier. Thank you for your help!


Although it is very difficult when you are trying to prove to the medic that your child has a problem, they are within range but low in the range. Trying to extract from a child how they are feeling , comparing and contrasting from a previous time is tricky. I think calculation and using the correct formula to the equation is important.


Ok. But just because the level is at a certain number doesn't mean that the child is feeling well. That's all I'm saying.


Yes I am with you totally but it is all about convincing the doctors where your child is in the range. My son has incredibly low FT4- rock bottom but still in the 'range". Doctors have no clue about being optimally that is where a formula/s could be useful.


Yes, but do you think they'd take any more notice of a formula than they do of the test result on their screen? Unless they come up with the formula themselves they're not going to recognise it. Doesn't matter how you calculate it, they Don't believe that T4 needs to be in the top 75%. They firmly believe that if it's in range, it's perfect. The range cannot lie.

To us it's obvious that if the result is at the bottom of the range, be it FT4, FT3, iron, B12 or whatever, that it's too low. But that's not what they learnt in med school so it doesn't exist. I would absolutely love for you to prove me wrong, but I Don't think you're going to. Good luck.


Seeing as there are so many doctors that like painting by numbers it is worth a go but as you say it is not what they learnt in med school so I am not holding my breath. Some doctors listen and take a little on board more than others, I will let you know how I get on.


Ok Good luck. :)


You could and should try to convince the doc that low in range can also give symptoms. If it will work? I don't think so but that doesn't mean you shouldn't try.

What helped me is talk to my GP and telling her that some thyroid patients told me about it and that as my level was just within range a trial with levo could be an option and I would like to try that. I got a whole lecture about the thyroid and that medication isn't good, blabla, and then I got it (!). Now my FT4 is high in range and my TSH lower than ever (was always too high) and I begin to feel something. I told my GP that and she was wondering if it then could be the thyroid that is irritating me for years (of course it is..). You see, it's never good (enough).

Go with your gut feeling, mine was right all the time but I thought the doctors would know best. I was wrong about that and now do everything myself.


Thanks Flower3, when there is a 'will' there is a way! Glad things are moving in the right direction for you.


Sorrel, I was calculating where in the respective ranges Flower and Anna are. Flower's FT4 and FT3 are in the top 75% of ranges and Anna's FT4 and FT3 are just below half way in her ranges.


Thanks greygoose, I have always been under the impression that our levels should remain within the upper third so you would multiply by 0.66. The equation I have followed was slightly different:-

Flower it can take a couple of months for your levels to stabilise, the hormone needs to get into the cells.


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Our levels should be where we feel comfortable. If we feel well with an FT3 just over the mid-range, then fine. But if we Don't, then it should be higher. You need what you need, and we shouldn't be restricted by so-called 'normal' numbers and ranges. But Don't quote me on the maths, because I'm hopeless! lol


Thank you Clutter. I can now see my levels are too low, which you were previously telling me. My FT3 is 3.9 and should be 4.64 and FT4 is 14.7 and should be 17.5.

My Endo does not want to raise T3, I'm on 10mg daily plus 100 Levo. I've just had RT3 done which is high at 30 (range 10-24).

GP wants to raise T3.

I think Endo waiting for result of Echocardiogram which has just come through as normal. for he is against raising T3 too fast in case of heart issues (I have a very low grade soft aortic murmur which I have had for 40 years)

I have raised T3 myself to 15mg and felt better.

How do I clear RT3? Is it dangerous to have RT3?

Hope you can help again Clutter.



Flowerpower, I made an error in the formula in earlier posts which I've corrected. Bottom range should be subtracted from top range, multiplied by % value and added to bottom range. I've corrected the errors and advised posters ie

FT4 22 - 12 = 10

10 x 0.75 = 7.5

12 + 7.5 = 19.5

FT3 6.8 - 4 = 2.8

2.8 x 0.66 = 1.84

4 + 1.84 = 5.84

'Ideal' ranges often don't take account of age, nutritional status and co-illnesses and should be viewed more as a guide to where you feel well than something to target. They can be useful to indicate possible undermedication or underconversion but it doesn't follow that one will feel marvellous once in those ranges.

rT3 is a build up of unconverted T4. rT3 is the body's defense mechanism against overconverting and becoming hyper. The usual way to reduce rT3 it is to increase T3 and substantially reduce or stop taking T4 until it clears. I hope the echocardiogram shows that it is safe for your endo to increase your T3. You'll find lots of posts if you type reverse T3 or rT3 into the HU search box and you may find something in Paul Robinson's posts


In my humble and non-math-brain opinion, it is possible to get hung up on the numbers and forget, as Greygoose has pointed out, that a lack of symptoms is FAR more important! :)

Please always also bear in mind that everyone is very different and some people may not actually need to be in that top 25% for any of the results! :)




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