What is the NHS FT4 reference range?: In 2014 I... - Thyroid UK

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What is the NHS FT4 reference range?


In 2014 I went to my GP eight times. I had a low TSH 1.1 and my FT4 was 12.9, falling to 12.3, 11.5, 10.8 and 10.1 throughout the year. It was at this point I asked to be referred to a private endocrinologist who diagnosed me immediately with secondary (central) hypothyroidism. He started me on levothyroxine and I immediately felt better but my GP has always refused to give me the levothyroxine on the NHS stating that I do not meet NHS criteria.

26 Replies


What is the NHS FT4 reference range?

There isn't a universal range, it varies from lab to lab even within the NHS.

The hospital lab that does my surgery's test uses 7-17 for FT4, but we also see 9-19, 11-23, 12-22 and others.

If you want to know the range, ask for a print out of your results or phone the hospital lab that did your tests.

Thank you, my GP’s lower criteria is 9. So does that mean that if I lived in different CCG area ie, one with the lower reference of 12 I would have qualified straight away?

SeasideSusieAdministrator in reply to Alipatsam

No. If you tested in a different area, because the range is different then your result would be different.

For example, if the range was 12-22 and your result was 13, you would be 10% through the range.

If you were tested somewhere else, you would still be 10% through their range, so if the range was 7-17 your result would be 8.

You can't take a result from one lab and put it with another lab's range. The equipment is calibrated with a range, your blood is tested with that equipment and your result is only applicable to that equipment and range.

Thank you for explaining that. I have been on 75mcg 4 days a week and 100mcg 3 days a week since January 2015 and am now symptom free. But in my pursuit of trying to force my GP to give me thyroxine I find myself with a difficult decision. I made a complaint to NHS England in December 2017 and it was agreed in a local resolution meeting with PHoWER that I should obtain a second opinion. My GP agreed to abide by the outcome and when the 2nd opinion came back with the same diagnosis, secondary hypothyroidism, the practice manager wrote to me stating that my GP was in agreement to add my medication to my records. He never did however and I contacted the Parliamentary Ombudsman who have just written to me stating that the GP under GMC guidelines does not have to prescribe anything he doesn’t believe in as he would be the one taking clinical responsibility.

So my options now are continue as I am paying for private consultations, medication and blood tests and remain well or go back to my GP and stop taking the thyroxine and wait until my FT4 falls to below 9 knowing that all my symptoms will return and I will feel ill. My third option was to move to an area with a lower range of 11 or 12 but you have explained this to me now and this is no longer an option.

SeasideSusieAdministrator in reply to Alipatsam

Crikey! What doesn't he believe in - secondary hypothyroidism or treating it? I wonder if there is any other condition that is so difficult to get diagnosed?

I don’t think he understands it. In my referral letter in December 2014 he wrote “Allison has read about a condition called Hypothyroidism Type 2 for which I can find little or no information”

With my second opinion in 2018 Professor Gittoes wrote “I explained to you this evening that secondary hypothyroidism is rare and the biochemical diagnosis is often indistinct. Patients with secondary hypothyroidism usually have a TSH within the reference range and a free T4 towards the lower part of the reference range (not necessarily frankly low). Your blood tests during the latter part of 2014, are compatible with a diagnosis of secondary hypothyroidism.”

To which my GP responded to me by saying “Secondary hypothyroidism is rare” “At the root of our decision is the lack of convincing medical evidence that you have either Primary or isolated secondary hypothyroidism. Your T4 has never been below the normal range, and TSH has not been abnormal”

SeasideSusieAdministrator in reply to Alipatsam

As secondary hypothyroidism can be due to either a problem with the hypothalamus or the pituitary, have any tests been one to look at these?

I had an MRI on my pituitary which showed normal size with no pituitary lesion. I had an IGF-1 28.6 (range 6.2 -24.3) and I had a synactin test which started at 348 (166-507) and went to 680 (166 -507)

SeasideSusieAdministrator in reply to Alipatsam

IGF-1 28.6 (range 6.2 -24.3)

What was said about this over range result?

My consultant wasn’t concerned by it. Professor Gittoes said it needed repeating. I haven’t seen my GP for over four years and I am self funding so I haven’t bothered

SeasideSusieAdministrator in reply to Alipatsam

Have you looked into what raised IGF-1 means?

It is a growth hormone

greygoose in reply to Alipatsam

IGF1 isn't actually Human Growth Hormone - although it is involved in the growth processes in children - it is an indicator of HGH levels. Like TSH can be used to diagnose thyroid problems, although it isn't actually a thyroid hormone. IGF1 is produced in the liver, among other organs, and it's production is stimulated by the production of HGH.

HGH is a pituitary hormone, but it's difficult to test for (don't know why), so they test IGF1 instead. Presumably, your HGH is also high, which could suggest a pituitary problem. But, other pituitary hormones should also be tested - and that needs to be done by an endo. The pituitary produces several hormones, and they could also be abnormal. Or, it could just be the TSH that is abnormal. But, you need to know, because it could affect your adrenals. Have you had any adrenal testing?

Low HGH can be caused by hypothyroidism, because you need good levels of T3 to produce the correct level of HGH. But, I don't think high levels of HGH would be caused by thyroid problems. But, I'm not an expert, and you should be taking this up with your endo. :)

Alipatsam in reply to greygoose

It is my FT4 level that I was having problems with. I was diagnosed with secondary hypothyroidism (pituitary) by two endocrinologists but my GP will not accept the diagnosis. Just looking back over my pituitary tests it was noticed that my IGF-1 was out of range. I have just emailed the practice manager to arrange a repeat blood test which hopefully may convince the GP to prescribe me on the NHS.

greygoose in reply to Alipatsam

You've had problems with your FT4 because you have problems with your TSH. If the pituitary doesn't produce enough TSH to stimulate the thyroid, then the FT4 - and FT3 (the most important number) - will drop lower and lower. But, I was saying that if you have a problem with your pituitary, then other hormones could also be too low, and therefore you need them tested - not just IGF1, which isn't even a pituitary hormone, but things like ACTH, which controls the adrenals. If the other hormones are low, they need to be replaced, just like T4.

Levels of IGF-1 that are higher or lower than normal may also be caused by: Hypothyroidism, or low thyroid hormone levels.

SeasideSusieAdministrator in reply to Alipatsam


Increased concentrations of GH and IGF-1 are normal during puberty and pregnancy but otherwise are most frequently due to pituitary tumours (usually benign). If other pituitary hormones are also abnormal, then the patient may have a condition causing general hyperpituitarism.

So is this linked to secondary hypothyroidism as it is the pituitary again? Obviously I need to get the IGF -1 blood test repeated. In January 2015 just before I started levothyroxine I had an IGF-1 blood test which was recorded normal at 24. The last test I had done was September 2018 at 28.6 and if it is high again what does that prove? And what treatment would I need?

SeasideSusieAdministrator in reply to Alipatsam

It can be.

Central Hypothyroidism refers to thyroid hormone deficiency due to a disorder of the pituitary or hypothalamus.

Secondary hypothyroidism is where the pituitary is at fault.

Tertiary hypothyroidism is where the hypothalamus is at fault.

A couple of articles here:

One that you can just see the summary but your GP will be able to see the whole article


and this one is much more involved:


if it is high again what does that prove? And what treatment would I need?

I don't know what it proves and I don't know what treatment would be needed, I'm not a medical professional and my own experience is Primary hypothyroidism (non-Hashi's), I'm just passing on information as you said you were diagnosed with Secondary hypothyroidism.

Considering that you have had this diagnosis, confirmed by a second opinion, and your GP originally agreed to abide by the outcome of the second opinion, then I am at a loss to understand why he has backtracked and then said that there was a "lack of convincing medical evidence" and I can't understand why he has to undermine the opinion of two endocrinologists who presumably have more knowledge of thyroid disease than your GP who is a generalist.

You have really given me some focus and direction tonight, thank you. I am not going to stop taking the levothyroxine. I will contact my GP and ask for the IGF-1 blood test to be repeated and take things from there. I will let you know how I get on.

Alipatsam in reply to Alipatsam

Just had my IGF1 blood test retaken and the result was 202.1

Just had my IGF1 blood test retaken and the result was 202.1

SeasideSusieAdministrator in reply to Alipatsam

I'm afraid I know nothing about this so I can't comment. As I said in a reply further up:

"if it is high again what does that prove? And what treatment would I need?

I don't know what it proves and I don't know what treatment would be needed, I'm not a medical professional and my own experience is Primary hypothyroidism (non-Hashi's), I'm just passing on information as you said you were diagnosed with Secondary hypothyroidism."

So you would need to discuss this with your doctor. If you want to know other members' experiences about this, maybe start a new post, making sure the title is relevant to this, as they wont see it in this thread.

There isn’t a clinical criteria for levothyroxine on the NHS, the guidelines suggest treatment should be initiated for a TSH over 10 or 12 but a doctor is free to use clinical judgement and prescribe for anyone over range if symptoms are present. I was prescribed based on a TSH of 17ish but my sister had symptoms and a TSH of about 6 or 7 and was also prescribed levothyroxine by her GP based on being over range and unwell.

From the BTA guidelines:

“There is no evidence to support the benefit of routine early treatment with thyroxine in non-pregnant patients with a serum TSH above the reference range but <10mU/L (II,B). Physicians may wish to consider the suitability of a therapeutic trial of thyroxine on an individual patient basis.”



The Clinical Knowledge Summary (managed by NICE but technically not a formal NICE Guideline) says this:

Scenario: Suspected secondary hypothyroidism

From age 18 years onwards.


If secondary hypothyroidism is suspected, refer urgently to an endocrinologist for specialist assessment of the underlying cause.


If being diagnosed with central hypothyroidism doesn't count as at least suspected, I really should be posting in Greek or Swahili - in English it seems absolutely clear.

GPs don't understand central hypothyroidism. If the endo says you have it, you should get an NHS prescription. TSH is irrelevant when you have central hypo, but the GP doesn't seem to know that and is implementing the guidelines for primary hypo (no treatment until TSH reaches 10, which yours never will). The GP should follow the endo's recommendations as in cks.nice.org.uk/hypothyroid...

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