How the TSH range is worked out
Has your doctor referred to the range given on your TSH test as the “normal” range?  Many doctors do, and I suspect that some even believe that to be correct – that if your TSH falls within the range it is “normal”.The range given is correctly referred to as the “reference” range.  Do you know how it is worked out?  I asked my local lab in Hereford just that.

They start by using blood that is sent in for testing and taken from patients who haven’t already been diagnosed with thyroid problems.  They told me they aren’t allowed to use blood from healthy volunteers.  I’m guessing that’s because it would cost too much to recruit volunteers and interview them to screen out those with possible thyroid symptoms.

So, they start with blood sent in for testing.  Why do most people have a blood test?  Because they aren’t well.

They find the mean (average) TSH of the tests sent in.  Then they apply a mathematical formula.  They apply two standard deviations to come up with a range that covers 95% of the population.  It’s nothing to do with whether patients are symptomatic,  it’s simply what is known as the 95 percentile range.  And it is used to try to ensure that no more than 5% of the population is diagnosed with or treated for thyroid problems.

This page explains how the tests are worked out in Australia.  The same way ours are.  and this is a quote “Reference ranges are defined as those into which 95% of a normal population fall. (Accordingly 2.5% of normals will have higher and 2.5% will have lower results than the reference range.)”

Do you know the TSH range of your local lab, that reference range that your doctor thinks is “normal”?  My local lab gives 0.25 – 5.0

Do you know the TSH range of a healthy population?

Do you know what the TSH range of a healthy population is?

This paper from the American Association of Clinical Chemistry states ” In 2002, researchers published an analysis of thyroid function test results from a large survey of individuals representative of the U.S. population (3). The study revealed that within a small standard error the mean TSH level in the general population is approximately 1.5 mIU/L.”

This study in The Journal Of Clinical Endocrinology & Metabolism states “Using the reference population, geometric mean TSH was 1.40 ± 0.02”

Whilst this study on PubMed found that healthy controls had a TSH range of “1.55 +/- 0.78”

In other words, the TSH range worked out by testing healthy people, or a very large sample of people is approx 1.5 with a small range of less than +/- 1.0.  Very different from the TSH range worked out by testing sick people and applying a mathematical standard deviation.

Many doctors, if you ask them, will claim that symptoms should be taken into account.  Yet few do.  Most look at the results that comes from the lab and go by the guidance written by the lab.  If the lab says the results are low or high your doctor might tell you.  Otherwise your results are all that the doctor will consider.  Your symptoms will be ignored.

Most labs won’t even test your FT4 level if your TSH is in the “reference” range.  How do they ever diagnose secondary or central hypothyroidism?

Central hypothyroidism is a pituitary  or hypothalamus led hypothyroidism. Your pituitary can’t produce enough TSH to stimulate your thyroid.  So when your TSH is measured it will be low.  How will your doctor diagnose you, as the lab will refuse to test FT4, let alone FT3?

Why Free T3 Needs Testing
You are welcome to print this out and take it to your doctor.  I wrote this originally on  Since writing this I have read a few practitioners who say that FT4 is used in the outer layers of cells, so my assertion that it is inactive may be wrong.

There’s no way of knowing if your TSH is “fine” or “normal” without knowing what your FT3 and FT4 are.  TSH on its own tells you nothing.

A quick lesson in thyroid hormones (very simplified). Your pituitary produces TSH (thyroid stimulating hormone). This does what it says on the tin – it stimulates your thyroid to produce 5 things, including T4 and T3.

T4 is an inactive hormone which is converted by the body into T3. T3 is the active hormone which is used by every cell in the body.

It is lack of T3 that makes you hypo and too much t3 that makes you hyper. This is why it is essential to measure FT3, that is the only reliable indicator of whether or not you are hypo.

You need to know FT4 also as you need to know if you are converting effectively from T4 to T3. If both are low in range, then you need more thyroxine (T4). If FT4 is high in range but FT3 low, then you are not converting properly and might benefit from the addition of some T3.

TSH is only useful once a patient has been diagnose as hypo. If it is high, then your pituitary is working properly and you have primary hypothyroidism.

If you are hypo (FT3 low) and TSH is low then your hypothyroidism is secondary (or tertiary) – you have an under performing pituitary or hypothalamus.

You can also refer your doctor to the Pulse online Continuing Professional Development module here .

This module clearly states that secondary hypothyroidism is characterised by TSH low/low normal and FT4 low.  Yet few doctors will even test FT4 if TSH is low, so most can never diagnose secondary hypothyroidism.Diagnosing secondary hypothyroidism