Always Tired? The Blood Tests to Ask For

If you are tired all the time and sleeping enough, the NHS route is a GP appointment where your symptoms determine the tests. A full blood count checks for iron deficiency anaemia and also picks up vitamin B12 or folate deficiency anaemia; a blood glucose test checks for diabetes; thyroid stimulating hormone checks for an underactive or overactive thyroid; and an antibody blood test checks for coeliac disease. Sleep apnoea is not a blood test at all — it is diagnosed at a sleep clinic. The order is set by what else you have noticed, not by a standard panel.

Last reviewed: 22 September 2026.

This page sets out what the NHS says each test checks, which cause it rules in or out, what a GP will ask you before ordering anything, and the wording the NHS uses for when tiredness needs help sooner. It is general information, not a diagnosis, and nobody here is a clinician.

Start with the symptom, not the test

The NHS publishes a symptom-to-cause table on its tiredness page. It is the most useful single thing to take to an appointment, because it turns “I’m exhausted” into a question a GP can act on.

What you have noticed alongside the tiredness	Possible cause, per the NHS
Tired during the day because of waking at night, making gasping, snorting or choking noises at night, loud snoring	Sleep apnoea
Lack of energy, noticeable heartbeats (heart palpitations), shortness of breath, pale skin	Iron deficiency anaemia
Very tired, feeling very thirsty, peeing more often than usual (particularly at night), weight loss	Diabetes
Tired all the time, finding it hard to stay still, feeling nervous, anxious and irritable, muscle weakness	Overactive thyroid (hyperthyroidism)
Extreme fatigue for at least 3 months making it very hard to do daily activities, problems with thinking, memory and concentration, flu-like symptoms	ME/CFS

Source: NHS, Tiredness and fatigue, page last reviewed 2 June 2023.

The NHS is careful to add: “Your symptoms might give you an idea of what’s causing your tiredness or fatigue. But do not self-diagnose – see a GP if you’re worried.”

It also lists causes that are not conditions at all, and which are far more common: not getting enough sleep or insomnia; an unhealthy diet and not much exercise; stress, depression and life challenges such as bereavement or looking after a new baby; hormonal changes during puberty, pregnancy or menopause; illnesses such as glandular fever and COVID-19; and some treatments and medicines, such as chemotherapy.

The tests, in the order the NHS logic puts them

1. Full blood count (FBC)

What the NHS says it checks: “Measures the number of blood cells in your blood to check for blood conditions such as iron deficiency anaemia.”

What it rules in or out: iron deficiency anaemia, and — importantly — vitamin B12 or folate deficiency anaemia at the same time. The NHS states on its iron deficiency page: “Iron deficiency anaemia is the most common type of anaemia. There are other types of anaemia, like vitamin B12 or folate deficiency anaemia, that the blood test will also check for.”

What happens around it: the NHS says your GP will ask about your lifestyle and medical history, and “You do not need to do anything to prepare for this test.” If the reason for the anaemia is not clear, “the GP may do other tests to find out what might be causing your symptoms. This might include having a faecal immunochemical test (FIT) to look for blood in a sample of your poo.”

Sources: NHS, Blood tests, reviewed 2 November 2023; NHS, Iron deficiency anaemia, reviewed 26 January 2024.

2. Vitamin B12 and folate

What the NHS says is checked: four things — “whether you have a lower level of haemoglobin (a substance that transports oxygen) than normal; whether your red blood cells are larger than normal; the level of vitamin B12 in your blood; the level of folate in your blood.”

The nuance most pages skip: the NHS adds that “It’s also important for your symptoms to be taken into account when a diagnosis is made. Some people can have symptoms and also seem to have normal levels of these vitamins. And some people may have low levels despite having no symptoms.” That is the NHS itself saying a number alone does not settle it.

What follows a positive: further tests may be arranged to find the cause, including checking for pernicious anaemia, an autoimmune condition in which you cannot absorb B12 from food. The NHS notes that “Tests for pernicious anaemia are not always conclusive.”

Why not to sit on it: “It’s important for vitamin B12 or folate deficiency anaemia to be diagnosed and treated as soon as possible. This is because although many of the symptoms improve with treatment, some problems caused by the condition can be irreversible.”

Source: NHS, Vitamin B12 or folate deficiency anaemia — Diagnosis, reviewed 20 February 2023.

3. Thyroid stimulating hormone (TSH), with thyroxine (T4)

What the NHS says it checks: “Measures thyroid hormones to check for conditions like underactive thyroid (hypothyroidism).”

What the appointment involves: the NHS describes it in full. A GP “will ask about your symptoms”, will “also ask if someone else in your family has had a thyroid condition or an autoimmune condition, or if you’ve recently been pregnant”, “may examine your neck to find out if your thyroid gland is swollen”, and you “may have blood tests to check how much thyroid-stimulating hormone (TSH) and thyroxine (T4) is in your blood.”

The threshold for asking: low. The NHS wording is simply “See a GP if you think you might have an underactive thyroid,” adding: “Some of the symptoms are common and can be caused by other conditions. But it’s important to get them checked by a GP.”

Sources: NHS, Blood tests; NHS, Underactive thyroid (hypothyroidism), reviewed 28 April 2025.

4. Blood glucose

What the NHS says it checks: “Measures your blood sugar levels to check for or manage diabetes.”

How diagnosis works: “Type 2 diabetes is diagnosed with a blood test that checks how high your blood glucose (sugar) levels are. The results usually take a few days.”

The symptom pattern that should trigger it: the NHS lists the most common symptoms of type 2 diabetes as “feeling very tired; peeing more than usual; feeling thirsty all the time; losing weight without trying to”, with blurred vision, slow-healing cuts and recurrent thrush as other possibilities. It adds that “If you get symptoms (not everyone gets them), they may develop gradually.”

Who is more likely to have it: the NHS names being white and over 40, or over 25 and from a South Asian, Chinese, Black African or Black Caribbean ethnic background; having a parent or sibling with it; and being overweight and not very physically active.

Sources: NHS, Blood tests; NHS, Symptoms of type 2 diabetes and how it’s diagnosed, reviewed 10 February 2025.

5. Coeliac antibodies

What the NHS says it checks: a “blood test to check for antibodies usually present in the blood of people with coeliac disease.”

The thing to get right before the test: this is the single most commonly botched step in the whole list, and the NHS is emphatic about it. “While being tested for coeliac disease, you’ll need to eat foods containing gluten to ensure the tests are accurate. You should also not start a gluten-free diet until the diagnosis is confirmed by a specialist, even if the results of a blood test are positive.” And: “You should include gluten in your diet when the blood test is done because avoiding it could lead to an inaccurate result.”

What a positive leads to: referral to a gastroenterologist, who “may arrange more blood tests, or a biopsy of your intestine.”

And a negative is not always the end: “it’s sometimes possible to have coeliac disease and not have these antibodies in your blood. If you continue to have symptoms of coeliac disease, despite having a negative blood test, the GP may still refer you to a specialist.”

The NHS also notes that after diagnosis “You may have more blood tests to check the levels of iron and other vitamins and minerals in your blood” — which is how coeliac disease and unexplained anaemia end up connected.

Source: NHS, Coeliac disease — Diagnosis, reviewed 31 March 2023.

6. Vitamin D — a supplement question more than a test question

Vitamin D is on every “always tired” checklist online. The NHS does not present it as a routine test for tiredness; it gives a supplement recommendation that applies almost universally in the UK, which changes what the test would achieve.

The recommendation, verbatim: “since it’s difficult for people to get enough vitamin D from food alone, everyone (including pregnant and breastfeeding women) should consider taking a daily supplement containing 10 micrograms of vitamin D during the autumn and winter.” Between late March or early April and the end of September, “most people can make all the vitamin D they need through sunlight on their skin and from a balanced diet.”

All year round, the Department of Health and Social Care recommends a daily 10 microgram supplement for adults and children over 4 who “are not often outdoors”, are “in an institution like a care home”, or “usually wear clothes that cover up most of their skin when outdoors”. The NHS adds that if you have dark skin — “for example you have an African, African-Caribbean or south Asian background” — you “should consider taking a daily supplement containing 10 micrograms of vitamin D throughout the year.”

And a warning that is missing from most supplement content: “Taking too many vitamin D supplements over a long period of time can cause too much calcium to build up in the body (hypercalcaemia). This can weaken the bones and damage the kidneys and the heart.”

One caveat on currency: this NHS page states it was last reviewed on 3 August 2020 with a next review due 3 August 2023. It is the current published NHS advice, but it is overdue for its own review, and we think you should know that.

Source: NHS, Vitamin D.

7. Sleep apnoea — not a blood test

If the answer is sleep apnoea, no blood test will find it. The NHS route is: see a GP with the symptoms, who “may refer you to a specialist sleep clinic for tests.” At the clinic “you may be given devices that check things like your breathing and heartbeat while you sleep”, usually done at home. The output is an AHI (apnoea–hypopnoea index) score: 5 to 14 is mild, 15 to 30 moderate, over 30 severe.

The NHS asks you to bring a witness: “If someone else has seen you have the symptoms, it can help to bring them with you to the GP.”

Source: NHS, Sleep apnoea, reviewed 11 May 2026.

8. ME/CFS — a diagnosis of exclusion

The NHS is direct: “There’s no test for myalgic encephalomyelitis or chronic fatigue syndrome (ME/CFS), but there are clear guidelines to help doctors diagnose it.” A GP should take a history and examine you, and “may also offer you tests like blood tests or urine tests to rule out other conditions, such as anaemia (lack of red blood cells), an underactive thyroid gland, or liver and kidney problems.”

It notes: “It can take a while for ME/CFS to be diagnosed because other conditions with similar symptoms need to be ruled out first.”

The NICE criteria, as the NHS states them, are that doctors should consider diagnosing ME/CFS if all of the following apply and are not caused by another condition: fatigue (extreme tiredness); sleep problems; issues with thinking and concentration; symptoms made worse by physical or mental activity.

Source: NHS, ME/CFS — Diagnosis, reviewed 28 May 2024.

What a GP will actually ask you

Nothing on this page replaces the consultation, and the questions are predictable enough to prepare for. Pulling together what the NHS says happens at these appointments:

Your lifestyle and medical history — named explicitly on the iron deficiency anaemia page.
Family history — the hypothyroidism page says the GP will ask “if someone else in your family has had a thyroid condition or an autoimmune condition.”
Recent pregnancy — also named on the hypothyroidism page.
A physical examination — the ME/CFS page says a GP “should ask you about your medical history and give you a physical examination”; the hypothyroidism page says the GP “may examine your neck.”
Your sleep, and what someone else has observed — the tiredness page’s own threshold for a GP visit includes being told you make “gasping, snorting or choking noises” in your sleep.
Weight and mood changes — again from the tiredness page’s GP threshold.

Worth writing down before you go: how many weeks or months it has been; how many hours you sleep and whether it is broken; what you have already tried; every medicine and supplement you take; and anything a partner has noticed at night.

Before the tests: the things the NHS says to try anyway

These sit alongside investigation, not instead of it.

From NHS, Tiredness and fatigue: have a healthy diet and exercise regularly; try to stick to the same sleep times and aim for 6 to 9 hours of sleep; try to relax a couple of hours before your sleep time; have a relaxing sleeping area that is not too bright, noisy or warm.

From NHS, Self-help tips to fight tiredness, reviewed 8 November 2023, whose section headings are themselves the advice: eat often to beat tiredness; get moving; lose weight to gain energy; sleep well; reduce stress to boost energy; talking therapy beats fatigue; cut out caffeine; drink less alcohol; drink more water for better energy.

If your tiredness comes with a headache on waking, waking with a headache every morning covers the shortlist of causes, several of which overlap with this page. If you work nights, the sleep itself is the first thing to fix — see how to sleep after a night shift. And if the pattern is specifically a mid-afternoon crash rather than all-day exhaustion, why you feel tired after lunch is a different question with a different answer.

When to see a GP

See a GP if (NHS, Tiredness and fatigue):

you’ve been feeling tired for a few weeks and you’re not sure why
your tiredness affects your daily life
you feel tired and have other symptoms, such as weight loss or mood changes, or you’ve been told you’re making gasping, snorting or choking noises when you’re asleep

See a GP if (NHS, Sleep apnoea) you have any of the main symptoms:

your breathing stops and starts while you sleep
you make gasping, snorting or choking noises while you sleep
you always feel very tired during the day

The NHS adds: “Sleep apnoea can be serious if it’s not diagnosed and treated.”

See a GP if (NHS, Insomnia, reviewed 19 March 2024): changing your sleeping habits has not helped; you’ve had trouble sleeping for months; your insomnia is affecting your daily life in a way that makes it hard for you to cope.

See a GP if you think you might have iron deficiency anaemia, an underactive thyroid, or a vitamin B12 or folate deficiency — the NHS uses that plain wording on all three condition pages.

What treatment looks like: the NHS says treatment “depends on the cause” and lists talking therapies such as CBT, advice on better sleep, and advice on relaxing or changing your lifestyle. It adds: “A GP may refer you to a specialist if they do not know the cause of your tiredness or fatigue. You may need tests, such as blood tests to check for anaemia, diabetes or hyperthyroidism.”

Urgent symptoms. Tiredness by itself is not an emergency, but some of the conditions behind it have their own red flags — and they belong to the symptom, not the fatigue. If tiredness comes with a sudden severe headache, weakness or numbness on one side, trouble speaking, chest pain or breathlessness at rest, treat that symptom as the emergency and call 999 or go to A&E. For headache red flags specifically, the NHS’s full 999 list is reproduced on our morning headache page.

What we could not source, and why it matters

Most “always tired” pages ranking in the UK publish a numbered panel — often “the 12 blood tests” — including ferritin, reverse T3, insulin and inflammatory markers. We have not, for a specific reason.

Ferritin. The NHS’s own blood test page does not list ferritin among common blood tests, and its iron deficiency anaemia page names the full blood count as the test a GP will usually do. Ferritin is a real and widely used measure of iron stores, but we could not source a patient-facing NHS statement about when it is requested for tiredness, so we have not invented a threshold or an ordering rule.

A standard fatigue panel. The authoritative UK primary-care reference on this is NICE’s Clinical Knowledge Summaries. Its tiredness topic is geo-restricted to users in the UK and is, in NICE’s own words, “third-party content hosted on the NICE site on behalf of the publishers Clarity Informatics Ltd” and “excluded from the NICE UK Open Content Licence”. We could not read it, so we have not paraphrased it, and you should be sceptical of any page that summarises a “NICE fatigue panel” without saying where it read one.

HbA1c. The NHS patient pages describe type 2 diabetes as diagnosed by “a blood test that checks how high your blood glucose (sugar) levels are” without naming HbA1c, so that is how we have described it.

Pages that publish a confident numbered panel are usually selling the panel. The NHS route is free, and the tests that matter for this question are on it.

Frequently asked questions

Can I ask my GP for a specific blood test by name?

You can ask, and it helps to explain why — which symptom made you think of it. What is tested is a clinical decision, and the NHS describes blood tests as being arranged when “a healthcare professional such as a GP, nurse or specialist thinks you need a blood test”. The NHS’s own symptom-to-cause table is the most useful thing to bring, because it links what you are experiencing to what would be checked.

Why did my GP not test my vitamin D?

The NHS does not present vitamin D as a routine test for tiredness. Its advice is preventive and applies to almost everyone in the UK: consider a daily 10 microgram supplement during autumn and winter, and all year round if you are rarely outdoors, cover most of your skin outdoors, or have dark skin. Because the supplement is recommended regardless, testing often changes nothing.

Should I go gluten-free while waiting for a coeliac test?

No. The NHS is explicit: “While being tested for coeliac disease, you’ll need to eat foods containing gluten to ensure the tests are accurate. You should also not start a gluten-free diet until the diagnosis is confirmed by a specialist, even if the results of a blood test are positive.” Cutting gluten first can produce a false negative.

My results came back normal but I am still exhausted. What now?

Go back. The NHS says a GP may refer you to a specialist if they do not know the cause of your tiredness or fatigue. It also notes for B12 and folate that some people can have symptoms with apparently normal levels, and that symptoms should be taken into account alongside the blood results when a diagnosis is made.

Is a private health-screening blood panel worth buying?

That is your call, but note two things. The tests that matter most for unexplained tiredness are available on the NHS, and a private result still has to go back to a GP for anything to happen. Private panels also often include markers the NHS does not use for this question, which can generate follow-up without changing the answer.

Can tiredness on its own be a sign of cancer?

The NHS does not list tiredness alone as an urgent red flag. It does treat tiredness plus something else as a reason to see a GP — specifically “weight loss or mood changes”. Unexplained weight loss alongside tiredness is exactly the combination to take to a GP rather than wait out.

How long should I wait before booking an appointment?

The NHS threshold is not a fixed number of months. It says to see a GP if you have been feeling tired for a few weeks and you are not sure why, or if your tiredness affects your daily life. If you also have weight loss, mood changes, or someone has told you that you gasp, snort or choke in your sleep, do not wait.