Thyroid UK
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Help with understanding ranges for tests, why they differ between labs and what this means for interpreting results and treatment

Hoping someone can explain something to me as I'm new to all this. Am starting to read lots and see lots of helpful things on this site which is great but can't find answer to one thing that I find curious. Why do ranges change dependent on the lab that's used to get tests done? Had one set of bloods done in November as part of a private gynea review, these were the results:

TSH - 4.21 range 0.27-4.2, FT4 - 11.1 range 12-22

Then had blood test early Jan with local GP and these were the results:

TSH - 0.95 range 0.34-5.5, FT4 - 9.5 range 7.5-19.0

So from first test to second test my FT4 has gone down (11.1 to 9.5) but doctor saying has gone from just below normal to well within normal because of the ranges. I don't get why the ranges change and how this affects interpretation of results.

In terms of how I'm feeling.

The gynea prescribed 25mg Levothyroxine. Started early Dec and after about 2 weeks felt some improvement for a little while, then plummeted back to being excessively tired, still putting on weight really easily, dry hair, mouth and skin, feeling cold. Some days better than others. My results dont look that bad compared to others I've seen here but really can't go on feeling this rubbish. Have more blood tests next week and am pushing to get T3 and antibodies done though GP thinks lab won't do it. Am also having ultrasound on neck as GP thinks thyroid feels slightly swollen and my throat is uncomfortable.

7 Replies

Symptoms, symptoms, symptoms. I feel very strongly, and this is borne out by numerous people I know, that if you have symptoms, regardless of what tests results say then you clearly still have a problem. 25mg of Levothyroxine may have been just enough to push your levels into an area that your GP wants to see, but that does not mean that you are where you want to be.

Perhaps ask your GP to insist on the paperwork that T3 and thyroid antibodies tests are carried out. That worked for us. Also ask for Vitamins B12 and D, Ferritin, Folates and Magnesium.

Once you have these results post them on here and you will have lots of suggestions about how to proceed.

Jane x


Thanks I'm getting a clearer picture now of what tests I need and will push as politely as I can at my next appointment If no success I'll investigate getting them done privately Thanks


When the first thermometer was made, the maker decided to calibrate it using a freezing point and the boiling point of water. Someone else, somewhere else, made their own thermometer and decided to do more or less the same.

One of those people was called Fahrenheit and he decided to use the freezing point of salty water and called that zero. And he set boiling point at 212. (Not as mad as it sounds, that makes ordinary water freeze at 32 and boil at 212, a difference of 180. So that is like degrees between North and South on a compass.)

The other person was called Celsius. He set the freezing point of ordinary water to zero. And boiling point at 100.

So we have two ways of measuring temperature which have persisted ever since. With freezing at 0 C or 32 F, and boiling at 212 F or 100 F.

(Please do not use the above to answer your A-level in History of Science.)

You could take a Fahrenheit thermometer and place it next to a Celsius thermometer and get a good idea about how the two scales compare and can be converted. But if the physical thermometers work differently, and the freezing and boiling points vary according to what is dissolved in the water, and atmospheric pressure, it is not a simple process to do accurately. You can easily imagine an ordinary mercury or alcohol thermometer where the tube up the middle is not even in size. That would make some gaps bigger than others.

In the same way, there are different technologies for measuring TSH, FT4 and FT3. And you cannot realistically have two sets of test results to measure in all circumstances. So we have imperfect tests and no simple way of making them all read the same, everywhere, all the time. (It is a LOT more difficult with medical tests than simple temperatures.)

All sorts of things can make one test read higher or lower than another. For example, antibodies to TSH itself will raise the TSH reading on some tests but not others.

Everyone, everywhere wants fully standardised, consistent and totally reliable tests. Even if discussions (or arguments) over choices of ranges continue, the actually numeric results would be the same. But they are not.

So all I can do is suggest that bottom of range from one lab represents the same situation as bottom of range at another lab – regardless of numbers. So instead of 32 and 212 on one thermometer, and 0 and 100 on the other, you simply have a blue line for freezing and a red line for boiling.



Thank you for taking the time to answer I kind of get it now There is a lot to take in when you're new!


Hi feeling better on a starting dose and then going " off" is normal as treatment should be increased ,with frequent tests until stable. you also need the Free T3 test for a proper diagnosis ( it lowers the TSH normally) as you may need T3 also, a lot of endo`s like this! I am stable, I still have an exceptionally low temp. One doc. did not believe this and it broke her thermometer, He! he!.

The reason for the different ranges is the assay`s are different at different Labs for lots of different blood tests ( I use 4, all different) but, so long as you always ask for the ranges with the results that is fine.



Thanks Jackie I think I will ask to have a copy of my tests results for my records Half the time I don't know what they've tested They just say all is normal unless I ask for specifics


Unfortunately, I have leanrt the hard way, never completely trust any doctor, even the best ones!

I hope you soon feel better.



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