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Rationalizing thyroid function testing: which TSH cut-offs are optimal for testing free T4?

Hands up - volunteers to be in the "minimal impact" group who are missed.

Hands up - those who believe that an FT4 test only has any relevance at that instant and does not contribute to the longer-term understanding of patients.

Hands up - those who believe the only real reason is saving money.

J Clin Endocrinol Metab. 2017 Aug 30. doi: 10.1210/jc.2017-01322. [Epub ahead of print]

Rationalizing thyroid function testing: which TSH cut-offs are optimal for testing free T4?

Henze M1, Brown SJ1, Hadlow NC1, Walsh JP1.

Author information

1 King Edward Memorial Hospital, Subiaco, Western Australia 6008, Australia; Department of Clinical Biochemistry, PathWest Laboratory Medicine, Queen Elizabeth II Medical Centre, Nedlands, Western Australia 6009, Australia; School of Medicine and Pharmacology, The University of Western Australia, Crawley, Western Australia 6009, Australia; Department of Endocrinology and Diabetes, Sir Charles Gairdner Hospital, Nedlands Western Australia 6009, Australia.



Thyroid function testing often utilizes TSH measurement first, then reflex testing for free T4 if TSH is outside reference range limits. The utility of different TSH cut-offs for reflex testing is unknown.


To examine different TSH cut-offs for reflex free T4 testing.

Design, setting and patients:

We analyzed concurrent TSH and free T4 results from 120 403 individuals from a single laboratory in Western Australia (clinical cohort) and 4568 Busselton Health Study participants (community cohort).


In the clinical cohort, restricting free T4 measurement to individuals with TSH below 0.3 or above 5.0 mU/L resulted in a 22% reduction in free T4 testing compared with TSH reference range limits of 0.4 and 4.0 mU/L; using TSH cut-offs of 0.2 and 6.0 mU/L resulted in a 34% reduction in free T4 testing. In the community cohort, the impact was less: 3.3% and 4.8% reductions in free T4 testing respectively. In the clinical cohort, using TSH cut-offs of 0.2 and 6.0 mU/L, elevated free T4 would go undetected in 4.2% of individuals with TSH 0.2-0.4 mU/L; in most, free T4 was marginally elevated, and unlikely to indicate clinically relevant hyperthyroidism. Low free T4 would go undetected in 2.5% of individuals with TSH 4-6 mU/L; in 94%, free T4 was marginally reduced and unlikely to indicate clinically relevant hypothyroidism.


Setting TSH cut-offs 0.1-0.2 mU/L below and 1-2 mU/L above reference range limits for reflex testing of free T4 reduces the need for free T4 testing, with minimal impact on case-finding.

PMID: 28938415

DOI: 10.1210/jc.2017-01322


26/09/2017 07:21

Of course, as in almost every paper which mentions TSH testing, time of day is 100% ignored.

Also, in this abstract, it appears that there is absolutely no possibility of FT4 being out of range if TSH is within range. Maybe the full paper explains more?

12 Replies

Much waving, all over the world!


Well, I can tell you for a fact that if FT4 is even 2 points above the reference range it is clinically relevant because it raises blood pressure and pulse and can make thyroid patients feel very unwell. According to this though the TSH cut off suggested would mean that it wouldn't be tested. This would mean a lot of thyroid patients would feel ill but not know why.

The people who make these decisions do not suffer from thyroid conditions. I think they should all be forced to live for 5 years with a virtual thyroid condition that gives them a myriad of symptoms and they should not be allowed any blood tests to ascertain what is wrong with them. See how they like it.


Kudos helvella for bringing a very crucial point forward .


So well said .

So this is what we are all up against and paying the price for our wellbeing ? What's wrong with them ? Who are they to decide what's stays what goes ? What's good for us and what's not ? Do they know what it means to have thyroid illness ? Maybe they should try it for size and then get back to us then .


Totally bizarre argument. We found that if we reduced the number of patients that were eligible for a freeT4 test, fewer patients would have freeT4 tests.


An average size shoe for a women is perhaps size (4-7)

This is my point. I am a size 5 the question is would I be happy wearing size 7's day & night, the answer is no. There is nothing like a range we are all different.

Hope you get it?


i AM ONE OF THE "LUCKY ONES" whose TSH NEVER rises above >.5. It usually reads .0005, NO matter what my thyroid levels are! My FT4 is usually .02 as I do not take any T4, and my FT3 runs 5.7 to 6.2 where I feel my best. It took my over 40 years of horrendous Hypo and bad doctors to find my magic numbers, and a TSH test is too unreliable to depend on for a diagnosis. Did you know that low cortisol or low Iron BOTH cause reduced TSH.?

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So you are one of the "minimal impact on case-finding" cases. :-(

It is utterly unforgivable to argue for a system which is bound to ignore cases like yours. Most especially as once the proposed system is implemented it becomes progressively more difficult to request further tests and less likely for them to be done even when requested. Exactly the way we have gone in the UK.

There are too many reasons for TSH to be out of kilter with actual hormones.


Most people and even VERY few doctors remember what the TSH was designed to do. I was diagnosed when I was 19 by an osteopathic doctor. He checked my reflexes which were non-existent. He took my temp, 97.0 in the middle of a hot West Texas afternoon. Prescribed me 1 grain Armour and told me to increase by 1/2 grain every 2 weeks till I felt well. There was NO TSH test then. Then 2 years later, my good doc died. I was farmed out to an Endocrinologist as I had abnormal thyroid issues. He did a TSH lab ONLY and said I did not need Armour, there was a MUCH better more stable brand new thyroid he would give me called Synthroid. The company that manufactures Synthroid, also manufactures and created the TSH lab to monitor SYNTHROID patients. Those of us that need extra T3 with the T4, or T3 only, or just higher than usual levels of T3, will always look hyperthyroid to those doctors that rely on the TSH. It was not meant to monitor people taking T3. T3 suppresses the TSH much more than T4 does as it is the active hormone.


A FT4 lab can also indicate other issues. FT4 above the mid-range of it's labs indicates a too high Reverse T3. Whether the high T4 causes the high RT3 can go either way. But when the body stops converting properly, standard medicine will fail us all the time.

1 like

What a load of utter codswollop!


Testing the TSH only, without testing the FT4 and FT3, is like me buying a jigsaw puzzle with 2/3 s of the pieces missing, and trying to fathom out what the bloody picture is! Now, I don't claim to be overly intelligent, but, my god, surely these " specialists " (cough cough) can work that out??? Where the hell is common sense these days 😡🙄



I have several time suggested that if TSH-only is such a good idea, someone should:

Collect sets of results from many patients - where they did actually do TSH, FT4 and FT3.

Hide one of the numbers in each set.

Get an endocrinologist to guess the blank.

Then repeat, but hide two numbers in each set.

I think we all suspect they whilst some blanks would fall within their expectations, others would be way out.

Obviously we can play all sorts of variations on this theme - select only patients who are undiagnosed, or who also have chronic kidney disease, or are taking certain medicines - or no other medicines.


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