How the "normal" range is set

It occurs to me that the "normal" ranges for thyroid blood tests must have been set by testing hundreds of "normal" people and getting the maximum and minimum from that. If they tested hundreds of people with "low" or "high" symptoms, surely that would produce a "low" range and a "high" range, which would overlap the "normal" range at each end by quite a lot.

Does anyone know how these "normal" ranges were produced?

Might this be a good argument for putting to your doctor?

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13 Replies

  • This is well commented on here and there, Chris. Whenever I raise anything contentious like this with a GP they just brush it aside implying thaey have too many concerns enough without taking on the 'system'. Quite right -we're measured against old and ill people and, what's more, maybe not even the same ethnic/DNA background.

    There are serious variations in genetic response in matters thyroid across the continents.

  • Chris, yes the ranges are based on population mean averages. Unfortunately, the population results will include people with undiagnosed hypothyroidism and probably hyperthyroidism and other illnesses and those on medications which may affect TSH. Add to that that TSH is likely to be as individual as finger prints the range is a very blunt tool indeed.

  • This is a post I did awhile ago about the subject. PR

  • Interesting post, thank you PR4NOW.

  • Hmmm. I read somewhere that they just used the tests from anyone who went for blood tests (that's generally just sick people) and then arbitrarily decided that only the top and bottom 5% were abnormal, and that it was a normal - not skewed distribution - with no scientific backing for the decision.

  • I read the same. If the test results are just from people who are not well and in need of blood tests to diagnose their illnesses, then the doctors are trying to get our results in line with sick people!!!!! That's pretty amazingly ridiculous!! So much for being scientific!

  • I read that the initial sample was quite small. The analysis was carried out by a statistician who decided that everyone within 2 standard deviations of the mean was normal, meaning that only the bottom 2.5% were hypo and the top 2.5% hyper. There was no attempt to relate the symptoms to the hypo or hyper groups.

  • In more recent years there has been significant effort to exclude anyone who has any hint of a thyroid issue. That is, anyone ever diagnosed, or with a positive antibody test, etc. This is one reason that the tops of the TSH ranges have, broadly speaking, dropped a bit within the past 15 years or so.

    Do remember that there are considerable numbers of people like blood donors, military personnel, insurance prospects, medical students,volunteers for medical experiments, etc., whose blood is tested when they are believed to be well. Such samples may well be available to be used in helping to determine reference ranges.

    Absolutely it is not a "normal" (also called Gaussian or bell-curve) distribution and no-one should ever treat it as if it were.

  • I heard the 'clinical tests' contained 12 people all over 60.....

  • Even if that were true for the very first test, and I don't know about that, your post seems to imply that absolutely no-one in the entire scientific and medical worlds has seen fit to look beyond that initial group in the many decades since. That is patently not the case.

  • During the thyroid summit, one Doctor said that they have seen the ranges, both the bottom end and higher end, lower by 15% over the years. I think he was talking about FT4 & FT3 ranges. So we are being told that people feel better in the upper ranges. But maybe these upper ranges are too low?

  • Without additional information we cannot be sure why ranges change. It is quite possible that improvements in the technology of a test change the associated reference range. So someone who was pretty good at, say, 90% of the way between lower and upper bounds, might still be quite happy at that point even if the range has changed markedly in numeric values.


  • No, nor has anyone asked why there are no clinical trials for levo, there never were any it was just shoehorned in because it was CHEAP and NDT which was the 'gold standard' of its time was discredited!!

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