I just replied in another thread saying that when I was first ill in 1976 my FT4 and FT3 were measured, but my TSH wasn't. I think this was St Thomas' Hospital in London, clearly the new TSH test hadn't quite taken hold there at that time.
It is widely stated that the usual reference range for TSH is the "normal" range. Normal has nothing to do with it. It isn't even how it is worked out.
This is how my local lab tell me the range is worked out. They take the average of the TSH from blood tests that are submitted, taken from people who do not yet have a diagnosis of thyroid problems. Bare in mind that most of those tests are taken from people who are unwell for some reason, and that they are not screened to ensure they are taken from people who do not have thyroid symptoms. The lab told me they are not allowed to take tests from healthy controls to determine the ranges.
So. They have these tests, taken from people who need a blood test. They test the TSH and determine the mean (average) value.
They then apply a mathematical trick. Not a medical test, not by questioning people to see at which level they start to feel hyper or hypo. No, a mathematical trick. They apply 2 standard deviations to ensure that a range is developed that covers 95% of the population from whom the tests were taken.
It is then randomly decided that those 95% cannot have thyroid problems and only the remaining 5% do. It is no coincidence that the incidence of thyroid disorder in most countries is 5%. Not because only 5% of people display thyroid symptoms, but because the artificially developed range only "allows" 5% of people to be treated.
(look at this too, australianprescriber.com/ma... ) Scroll down to "Reference Ranges".
Compare that with this study ncbi.nlm.nih.gov/pubmed/157... which found a TSH in healthy controls of 1.55 +/- 0.78
Of course the above applies to people with primary hypothyroidism. The TSH test is worse than useless for anyone with secondary/tertiary hypothyroidism or who has a conversion or uptake problem.