Blood test range change

[Andie222]

Hello

I’ve just had an NHS blood test for thyroid (TSH, FT4 and FT3) in Bristol. It seems the FT4 range has changed considerably since my last test. It used to be 12-22 and is now 7.9-14.4, so my 14.9 is over range. In 2 months I’ve gone from 67% of old range to 107% of the new one. Testing was at the same time. No changes except from a different brand of T3 so I was rather surprised by the results. Can anyone else in Bristol area confirm these ranges are correct please?

[PurpleNails Administrator]

It’s not that different areas set different ranges it’s that lab machines are calibrated differently so in theory if you tested using both machines, the % should be the same.  (Although there’s likely a slight variation) 

I’ve seen both those ranges used at different times on my own results from the same location.  

What was FT3 & TSH.

[tattybogle]

See this post and the replies to it ,

healthunlocked.com/thyroidu... nhs-bristol-changed-their-t4-range-to-7-9-14-4-pmol-l

The range has changed because the lab are now using a test platform from a different manufacturer which is calibrated slightly differently because it uses a different process .

In theory this should not change the '% through range' of your fT4 results , eg,

a blood sample that gave a result of 17 [ 12-22] 50 % through range , on the old machine

ought to give a result of about 11[7.9 -14.4] also 50% through range.

( its not just the range that changes , the result does too)

However , the only way to be sure if this IS happening in practice , is to test 2 samples from the SAME blood draw on both types of machine and see if the results really are comparable ,or if the new test machine is giving a higher % though range .

If you are concerned the new machine is giving you a significantly higher % through range, then ask GP to query it with the lab.

my lab has used 7.9-14 range since 2017 , and i do have some concerns that my % through range is now higher ,but can't be certain as my fT4 results have always swung about wildly anyway.

[tattybogle in reply to tattybogle]

healthunlocked.com/thyroidu... varying-lab-ranges-accroos-the-counry

healthunlocked.com/thyroidu... tsh-range-altered-from-last-time

healthunlocked.com/thyroidu... a-critical-need-for-ft4-test-standardisation

healthunlocked.com/thyroidu... free-t4-reference-intervals-ft4-ranges

[Andie222 in reply to tattybogle]

Thank you tattybogle. I'll have a look at the links. I don't often test so close together without having made some change to dosage, so I've never had reason to suspect an error before. On reflection the only thing I changed about a week previously was to reduce my HRT dose, but I can't believe that would explain the difference!

[tattybogle in reply to Andie222]

Theoretically , HRT dose change could affect fT4 level ... (i don't know how large an effect , or how long it would take)

Estrogen level affects the amount of TBG (Thyroid Binding Globulin) that the liver makes.

TBG is what binds T4 .

'unbound' T4 is 'free T4'.

if you change the levels of TBG in the blood , you would get different proportions of bound : unbound T4 .

which would lead to FT4 changes without any change in the total level of T4 (TT4)... and with out any dose change .

however , my understanding of this is very sketchy ........

[Rollercoaster44]

Yes bristol have changed their ranges in nov last year….nbt.nhs.uk/severn-pathology...

[Andie222 in reply to Rollercoaster44]

Thanks Rollercoaster44