Thyroid UK
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Ranges question

why do different people have different ranges for their test results within the same country (UK)? Surely if the units of measurement are the same then the ranges should corrolate??

Does someone with a lower TSH in London have thyroid disease if their results are low due to a different range of someone in the north??

Is thyroid disease geographic like the different ranges suggest?

12 Replies

No - the variation in ranges has almost nothing to do with geography.

If you drive two different cars, their speedometers are likely to show different speeds. One might be spot on. Another over-reading by up to 10%.

So in one car, 70 mph on a motorway is the speed limit; in the other car the speedo might show 77 mph when it is actually travelling at 70 mph - same as the first car.

This is not how anyone wants the world to be. But when the machines and everything to do with testing are made by different companies, using different technologies, this is what happens. Even then, if it were simple to correct, then that would happen - it is not.

Basically, all we and doctors can do is take the bottom of range as equivalent on all tests; and similarly the top of range.

So, take two labs, one has FT4 range 6 to 14; the other has 12 to 20.

6 at lab1 should mean something similar to 12 at lab2.

14 at lab1 should mean something similar to 22 at lab2.

And, at a guess, 10 at lab1 should mean something similar to 16 at lab2.

And it could be that lab1 and lab2 cover very similar geography (e.g. two nearby labs in London), or they might be far apart as in Aberdeen and Exeter. Makes no difference. But labs do make comparisons between tests on "healthy" patients to make sure their results are believable. So there can be a tiny bit of geography in there.

A while ago I posted a blog which shows how variable different machines can be - though in a special case:



So if the cars are both going at 70 miles per hour, how come 6 at lab 1 is bottom of the range but it would be -6 from the bottom of the range in lab 2, isn't the hormone/blood ratio still the same if the units it measured in the same?


Sorry - I have completely failed to explain. Perhaps someone else can think of a way of explaining?

Just accept what I said about bottom (or top) of range at lab1 and lab2 having different numbers but meaning the same.

So you cannot readily compare results from different labs by saying "Ah - it was 10 last time but it is 12 this time so it has gone up" if the results were from different labs with different ranges.


I'm sorry Rod, I'll give you the working example that has puddled me. I have a FT3 result from 2010 of 4.1. I found the range for FT3 on thyroid uk to be 4 - 8.3 which would put me on the edge of normal.

However, when I asked my doctor about this she said the range from the lab was 3.1 - something (can't remember) which made my result not on the edge of normal.

So does that mean my FT3 is ok or does it just look ok because my docs range is different from thyroid uk??


The ONLY range that has any relevance whatsoever for your test is the one that was in use at the lab and at the time at which your test was processed.

The TUK site says:


There is a range, which is used for the tests. These vary sometimes which is why you must always ask what the range is, so that you can see where you are in the range.

The example ranges there really are only to give you some idea of what to expect and must NEVER be used for interpretation.

If the range from the lab was 3.1 to <something> then indeed, 4.1 is not right on the edge. Though that does not mean it was OK - I suspect it was a little lower than optimal but would not go further without knowing a lot ore.

However, a three-year-old FT3 blood test is, pretty much, irrelevant now. FT3 can vary quite quickly within a day and between one day and another.


I'm just not getting this, so sorry if I sound dumb, but if your sample is a measured unit of 4.1 pmol/L, isn't that going to remain the same no matter what the range is?

1 like


What I always tell people on the phone is that the figure is less important than whereabouts in the range you are.


T3 5 (1-5) top of the range - probably feeling ok

T3 5 (5-10) bottom of the range - probably feeling rubbish

Hope this helps in some way..!?




Hi mitchell66, I asked the same question several weeks back but didn't get a response.

Like you I am also completely confused.

I totally agree with your above comment if a measured unit is 4.1 pmol/L it should surely be the same whoever does the test.

There seems to be 2 parts to this conundrum, the result and the ranges. The resultant measurement should remain constant within a sample, whilst ranges can be variable dependent on the parameters used.


When analytical techniqes are so sophisricated now able to measure ppm. then how the test is done the results should be consistent.

If you have a salt solution of 10mg/L it should be that result wherever and however the tests are performed..

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another example siskin is that the USA range for TSH is 0.3 - 3 and my doc reckons she works to 0.3 - 6. Therefore someone with a TSH of 4 is hypo in America but normal in UK.

I just can't help wondering, if the sample is measured in the same units wherever it falls on whatever range it's still the same pmol/L (or which ever unit the sample is measured in).

Maybe it's one of them things I should just except ;)


I cannot understand why there is such a difference in ranges used by the labs. Why is there a difference?? There shouldnt be.

My aunt lives in Barry Island in Wales, and she said that her TSH range there is 0 - 4 and that she has asked her very understanding GP for advice about me, and he said that as my TSH was 3.89, then I would have been given levo, yet the endo I was sent to here in Wigan, said that my TSH would have to get to nearer 10 for me to get treatment!!!

So unfair, and it is costing people their health and lives!

Ann xx


But 10 is not the top of a TSH range - it is the doctor's decision that he will not treat until it gets to that arbitrary number.

I was started on treatment pretty much as soon as my TSH went out of range. Which shows that practice varies from one doctor to another.

I agree 100% that refusing to treat between some degree of elevated TSH (even within range) and 10 is a hopelessly unacceptable approach. But it is not a matter of laboratory ranges as such.


Hi Chemical Angel. As I see it the ranges are based on statistical analysis and can vary according to the criteria used.

An example could be say, the average height of dogs If someome measured the height on a 100 random passing dogs it would give one result.

If however a sample of 100 dogs was was studied at say Crufts you most likely get a different result.

In the random sample you probably would not measure an Irish Wolfhound or Chiwawa (sp?) but in the Crufts sample they would be includeded.

This would result in a different average height of dogs.

The range for Crufts would probably 8" to 6 ft. the random range would be different say, 1ft to 4ft 6"

Hope this helps.

Figs. given are only example, and bear no relation to actual measurements.


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