T3 and T4 ranges - VERY PUZZLED PATIENT - Thyroid UK

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T3 and T4 ranges - VERY PUZZLED PATIENT

fiftyone profile image

Can anyone explain the ranges:

1. On what are these ranges based?

2. How can the ranges differ so widely in different areas. Surely the range is the range?

3. Ten years ago, top of the range in my area was 26. Today it is 14.7. How can this happen. Surely the repurcussions are enormous.

4. My body is not going to change because Joe Bloggs has decided to bring the range down ten points. My cells are not going to suddenly decide they need a whole lot less thyroxine because the range has been reduced overnight. How does the medical profession work with this?

34 Replies

I was wondering this too. It seems outrageous!

When a test is manufactured and developed by a particular company, they have to produce a "reference range" which is supposed to describe the range of values healthy people show. For FT4 and FT3 this is usually a factor of two between the lowest values and the highest. The real problem now comes up as to how carefully they calibrate the test. That is, what actual numbers describe the top and bottom limits. The scandal is that there is no mandatory way to do it. So one company can come up with a range that is quite different from another ,given the same patient group. So suppose you are tested by a test with a range 9-22 (FT4) and you come out at 17. You are in the middle of the range. Now lets go to another test from someone else with a range 7-17 say. Now your number will be 13. The answer therefore is that various manufacturers have different ranges, which are now at this moment trying to be reconciled with better calibration. But within each range your result is 77% of the top end in each case even though the numbers are quite different. So it isn't the actual number that matters but your position in the range typical of the product. This is a joint failure of manufacturer competence, regulatory failure, and medical indifference.

fiftyone profile image
fiftyone in reply to diogenes

so when I have my blood tested for T3 and T4 they aren't actually measuring what's in my blood. If they were it would be the same whether it was taken in Scotland of Cornwall. If T4 was 17 in Scotland it would be 17 in Cornwall, surely, regardless of the area range or the health of the local population.

diogenes profile image
diogenes in reply to fiftyone

Yes they are testing what is in your blood. But the problem is that the actual numbers can vary test to test. BUT your position of values in each test will be the same. It's where you are in the test results that matters, compared with the range limits, not the actual number itself.

fiftyone profile image
fiftyone in reply to diogenes

Still puzzled. If my blood was tested in my home town and the level of T4 found was 17, how can the level in my blood change, just because I've moved to another town? Surely it's the ranges which change from area to area, not the amount of thyroxine in my blood???? When the top of the range was 18, my T4 was 22. Does this mean that now the the top of the range has been reduced to 14.7, my T4 level at next test will automatically drop by four points to around 18? I would have thought the only way to reduce the T4 in my blood is to take less thyroxine, not just move to another area with a different range. Surely it's what is actually in your blood - and, MORE IMPORTANTLY, how it affects you - which is more important than where you are in the range.

RedApple profile image
RedAppleAdministrator in reply to fiftyone

There are two things going on. Firstly, the level in your blood naturally fluctuates all the time because the T4 is being used by the body, and according to how long between taking your thyroxine and having the blood drawn.

The second thing is the equipment used to do the testing will vary. A simplistic example might be to say that you have two pairs of scales next to each other. You stand on one and it says you weigh 65 kilos. You step off and stand on the other and it says you weigh 68 kilos. You know your weight hasn't changed in those few seconds, but the scales give different results.

This is why, where you are in the range for the equipment that did the test is more significant than the actual number.

Does that help?

fiftyone profile image
fiftyone in reply to RedApple

And what if I stand on the scales and weigh 65 kilos every time, but because the range has changed, one time I seem high, next time I seem normal? But I am still 65 kilos??

RedApple profile image
RedAppleAdministrator in reply to fiftyone

Nothing can be taken in isolation. Your T4 level will vary according to time of day, and it will be lower twelve hours after you took your thyroxine than it would have been two hours after taking it. The range for the actual machine that did the test is constant.

So get your test done at the same time of day, and by the same lab. This is the nearest you can get to an accurate reading for the purposes of judging your medication dose. If you change the time of the test, and/or the lab equipment, then you can expect a different reading.

This reading does of course have to be taken in conjunction with symptoms. If you feel good, then the test result is a secondary issue (although not all doctors see it that way of course and some would rather keep you unwell than change your dose!)

helvella profile image
helvellaAdministrator in reply to fiftyone

Imagine measuring your height with a tape measure.

Then imagine measuring the heights of all the adults in your town with the same tape measure.

You will (almost certainly) end up with a nice graph (like a bell) showing the distribution of heights.

Now repeat at the next town down the road. BUT you can't take the tape from first town to second town every few minutes! So second town uses their own tape. Trouble is, that tape doesn't measure quite the same as the first tape. Over the years one tape has been crumpled up, and shrank when it got wet, and it came from a different factory. Whereas the other tape was kept carefully, but has worn thin and become somewhat stretched.

Both sets of results will look identical when plotted as graphs. If you are in the exact middle in the first town, you'd expect to be in the middle point in the second town (unless there is something very different about the people). But the numbers might be different. One might have you at 151.6 cm and the second at 171.6cm.

Both measurements properly represent where your height lies with respect to other people. If you are taller than Ann in the first town, and you both go and get measured in the second town, you will be taller than Ann there as well.

The problem only really comes about if you take your measurement in the first town and try to fit yourself into the graph at the second town using just the number from the first town. If you either get measured again in the other town OR you place yourself in the middle of the graph all would be well.

fiftyone profile image
fiftyone in reply to helvella

It strikes me that the sample of people on which the ranges are based is the key to this system, and whether the sample has a similar metabolism/ physiology to myself. If the sample is similar then I will appear 'normal'. If not then where I stand in the range will be abnormal. And if a new sample is taken with a different set of individuals, then this, again, is going to affect my position in the range. Is this anywhere near correct??

helvella profile image
helvellaAdministrator in reply to fiftyone

My point was that ALL testing/measuring systems are imperfect.

Tape measures can vary one from another.

Of course, in my example, one town could be full of people renowned for being tall (basketball players) and the other full of people renowned for being short (horse or camel racers). But I expressly added (unless there is something very different about the people) - meaning, where the people are a very similar mix in both towns.

Although lots has been said about adjusting ranges to local populations, I have also seen it said that some laboratories simply use the manufacturers' ranges without any attempt to tailor them to their populations. I highly recommend that you ignore the possibility of tailoring to local populations. The important issue is that there are differences between the measurement techniques. and most of that appears to come from the actual technology used by the manufacturer.

fiftyone profile image
fiftyone in reply to helvella

If the range changes significantly, say top T4 is 20 one month and changes to 15 the next, is that an indication that the physiology of the sample has changed dramatically and/or the equipment used is very different??

helvella profile image
helvellaAdministrator in reply to fiftyone

The most likely answer is that it was a different manufacturer or different model of the test machine/kit.

If you actually go round looking at the websites for testing laboratories, you will from time to time see notices saying things like:

Please note, the range for Free T4 tests will change as of 1st October 2018.

Laboratories do have to buy new machines. At the moment there seems to be a move towards merging laboratories into super-laboratories to reduce costs (they imagine). It is entirely feasible that what looks like one laboratory actually operates two sites and each could use a different machine. Your blood could go to either. The range would depend on the machine used.

But these changes do not occur day by day, week by week, month by month, nor even year by year. Typically they are every few or many years.

fiftyone profile image
fiftyone in reply to helvella

When I look back at my records, ten years ago the top T4 range was 26 and I Today, the top range is 14.7 , down from 18, which itself was a reduced only three years before from something even higher. That seems incredible that the range should alter so much.

Pascha1 profile image
Pascha1 in reply to diogenes

My Question is T3 ranges

T3 are tested from so called normal people,?

I find this hard to understand how they have tested so called normal people with a T3 test, this is very hard for even thyroid patients to get a test done for T3 !

T3 certainly is not done in routine testing as GPs or many other Drs dont seem to be able to understand the results that seems why they do not test I have asked severel and told The T3 was not a reliable way to test thyroid as it goes up and down too much !

So my point is how on earth have they come up with a normal range for T3 as far as i have heard this is near on impossible to get tested unless you are ill and beg for it, it may get tested in mental illness and Cardiac issues so I how would thisthis be a good judge of a normal T3 ranges are ?

it is not tested as routine on anyone ill , let alone normal people ! ?

diogenes profile image
diogenes in reply to Pascha1

Before ANY blood test can be issued on to general use, we have to know what are the figures and the spread of those figures for normal healthy people. Such people are usually the lab staff in the development company and other volunteers perhaps in the company as well. When we developed tests for FT3 and FT4 in the company I once worked for, the lab staff all went down to the practice nurse to give an armful of blood to produce the normal ranges. I was one to get the horse-needle treatment. So I know I'm right in the middle for both tests. With all the figures, you then perform simple statistics to get a normal range that includes 95% of people, with 2.5% outside. That may seem strange, but it minimises the overlap of especially hypo and normal, and hyper and normal values at each end of the range.

Pascha1 profile image
Pascha1 in reply to diogenes

Thank you,

What were your thyroid blood results out of interest and are you normal not hypo ?

Marz profile image
Marz in reply to Pascha1

diogenes is an Independant Analyst - click onto his name for more information and the research he is involved with. His wife is a thyroid sufferer - he is is a scientist. Aren't we fortunate ? :-)

Pascha1 profile image
Pascha1 in reply to Marz

Ahhh I see Thank you . so well trusted information :)

Marz profile image
Marz in reply to Pascha1

ABSOLUTELY !

Pascha1 profile image
Pascha1 in reply to Marz

Thats good to know :)

Marz profile image
Marz in reply to Pascha1

if you click onto his username above - it will take you to all his posts and replies - have you ever done that ? There is also an e-book that you can download in one of his posts with all the up-to-date research papers he has been involved with. Way ahead of our current treatment models ....

Pascha1 profile image
Pascha1 in reply to Marz

Thank you :)I will do this x

diogenes profile image
diogenes in reply to Pascha1

My wife has questions about my normality, but thyroid wise very normal and ordinary in the range middle.

Pascha1 profile image
Pascha1 in reply to diogenes

Ha ha & Thank you :)

SeasideSusie profile image
SeasideSusieAdministrator

fiftyone

Simply -

1) Ranges are based on local population, 95% of the local population would fall within that range

2) See answer to (1)

3) It doesn't matter what the top of the range is, if it was 26 before and 14.7 now, I doubt very much whether the bottom of the range has stayed the same, that would have changed too. Most ranges for, say FT4, have a width of around 10 to 12, for example my lab's range is 7-17 (used to be 11.8-24.6), we see 9-19 and 12-22 here usually. So you take where you are in the range when comparing, eg if the range is 12-22 (Medicheck's range) and my result is 17, I am 50% through the range. My GP's range is 7-17 and if my result is 12 then again I am 50% through the range.

4) See answer to (3) - the bottom of the range will usually change as well, and it's where you are within any given range that matters. This is why you can't use Patient A's result with Patient B's reference range unless you know the same range has been used.

Which is your area?

fiftyone profile image
fiftyone in reply to Ginny52

Cumbria

Ginny52 profile image
Ginny52 in reply to fiftyone

Thanks! The hospitals my gp can send me to are all within imperial’s group. I found a document online with all their ranges and I have to say they’re absurdly wide- how can x be fine, when 100x is fine too? (Tsh, but others too)

ranges are based on where 80-90% of the population are okay - statistically 'normalised'

ranges will depend on

a) what units are being use (different units can be used for tests - generally the difference between a measure of the number of molecules and a measure based on the mass - roughly the weight) of the product, or the volume.

b) different pieces of kit may measure differently so may have different ranges - don't know if this is a factor for thyroid related results.

c) ranges can also be varied by the lab to reflect the accuracy of the measurement and whether it could be picking up other molecules as well as the one being formally tested.

LAHs profile image
LAHs in reply to Gambit62

Yes, Gambit you took the words out of my mouth. Here in the US we use ng/dL (or pg/mL) whereas units I see quoted a lot in the UK are nmol/L (or pmol/L). We do, however, both seem to measure TSH in mIU/L.

So fiftyone, check that the units are the same first, and then apply all of the advice you have been given above.

helvella profile image
helvellaAdministrator in reply to LAHs

There is a fundamental technical reason for measuring milli-Units of TSH.

What is measured for TSH is how much effect it has on the immunoassay kit. And that depends on number of molecules AND its precise form. TSH molecules themselves need not all be the same weight. Each molecule of TSH has sugar molecules attached to it, it is glycosylated, and the precise details vary. There are different variations of TSH - called isoforms.

(The effects of different glycosylation of TSH is a field that has not yet been ploughed.)

Any attempt to convert back from measuring impact of TSH to weight (or mass) would be scientifically unjustified.

To illustrate moles (number of molecules) and weight (mass) using another thyroid example:

One mole of levothyroxine (T4) weighs 776.874 grams.

One mole of liothyronine (T3) weighs 650.977 grams.

The difference is due entirely to the fact that each molecule of levothyroxine has four iodine atoms; each molecule of liothyronine has three iodine atoms.

To put that in more usual units and context, 100 micrograms of levothyroxine (T4) converts to about 83.8 micrograms of liothyronine (T3). But both have exactly the same number of molecules.

[ 27/09/2018 19:14 - Just editing this post to correct the first bit about TSH where I have mis-represented the details. LAHs - please put the updated version on your fridge. :-) ]

LAHs profile image
LAHs in reply to helvella

Gee thanks! I have never really had all that straight, thanks for explaining. That's a post I will print out and stick on my 'fridge. Excellent explanation, thanks.

LAHs profile image
LAHs in reply to helvella

That's OK, I thought you meant that.

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