Why you are always tired, and which blood tests matter

If you are always tired, a good GP work-up should start with the pattern of fatigue, red flags, medicines, sleep, mood, exertion, periods, weight change and a focused set of blood tests. It is reasonable to ask about full blood count, ferritin, thyroid, kidney and liver function, glucose or HbA1c, inflammatory markers, coeliac testing and other tests that fit your symptoms. It is less useful to demand every private panel at once, because untargeted testing can produce false positives without explaining why you feel exhausted.

Why GPs do not run every test

Fatigue is one of the hardest symptoms in primary care because it is common, personal and broad. It can come from anaemia, thyroid disease, diabetes, infection, inflammatory disease, coeliac disease, kidney or liver disease, pregnancy, cancer, sleep disorders, depression, anxiety, burnout, medication effects, pain, overtraining, under-fuelling, long COVID, ME/CFS or ordinary life overload. It can also come from several small problems at once.

That breadth is why testing has to be targeted. A broad panel feels reassuring, but every extra test adds a chance of a mildly abnormal result that is not the cause. That can lead to repeat testing, referrals, anxiety and still no answer. A systematic review of tiredness in primary care found wide variation in diagnoses and stressed the challenge of distinguishing self-limiting fatigue from specific medical causes.³

This does not mean "no tests". It means the best testing starts with a clinical question. Are you tired because oxygen delivery is poor? Check anaemia and iron. Because metabolism is slow? Check thyroid. Because blood sugar is abnormal? Check HbA1c. Because inflammation is present? Check inflammatory markers and symptoms. Because sleep is fragmented? Blood tests may not be the main tool.

First-line tests that are often reasonable

NICE ME/CFS guidance is useful even when ME/CFS is not the likely diagnosis because it lists investigations that may help exclude other causes of fatigue. These include urinalysis, full blood count, urea and electrolytes, liver function, thyroid function, ESR or plasma viscosity, CRP, calcium and phosphate, HbA1c, ferritin, coeliac screening and creatine kinase where appropriate.²

Not every person needs every test on day one, but this list gives a sensible conversation starter. A full blood count can identify anaemia, infection clues or blood disorders. Ferritin can show low iron stores before anaemia is obvious. Thyroid testing can identify underactive or overactive thyroid disease. HbA1c can screen for diabetes or prediabetes. Kidney and liver tests check organ function and medication effects. Coeliac screening matters if fatigue comes with gut symptoms, iron deficiency, weight change, mouth ulcers or family history.

Iron deserves special attention. NHS guidance lists tiredness, lack of energy, shortness of breath and noticeable heartbeats among symptoms of iron deficiency anaemia.⁴ Heavy periods, pregnancy, recent birth, vegan or vegetarian diets, endurance training, gut symptoms and blood loss can make iron deficiency more likely. Ask about ferritin, not just haemoglobin, if low iron stores fit your story.

Thyroid disease is another common worry. NHS guidance says symptoms of an underactive thyroid can include tiredness, weight gain, sensitivity to cold, constipation, depression, slow movements and dry skin.⁵ A normal thyroid-stimulating hormone usually makes significant hypothyroidism less likely, but interpretation depends on pregnancy, pituitary disease, medication and previous thyroid treatment.

Tests that need a specific reason

Some tests are not usually helpful as broad fatigue screens. These include reverse T3, large sex-hormone panels in people with regular cycles and no relevant symptoms, random cortisol tests without adrenal features, broad autoimmune panels without joint, skin, kidney, neurological or inflammatory signs, tumour markers without a suspected cancer pathway, viral antibody panels that only show old infection, food IgG intolerance tests, and commercial microbiome tests.

That does not mean these areas are never relevant. Cortisol testing may be important if there are symptoms of adrenal disease such as unexplained weight loss, low blood pressure, salt craving, skin darkening, vomiting or severe illness. Sex hormones may be relevant with missing periods, early menopause symptoms, infertility, galactorrhoea, PCOS features or low testosterone symptoms in men. Autoimmune testing may be relevant with swollen joints, rashes, mouth ulcers, Raynaud's, kidney findings or abnormal inflammatory markers.

The question is: what decision will the result change? If no one can answer that, the test may not help. If the result would change treatment, referral or monitoring, ask the GP to document why it is or is not indicated.

What normal blood tests miss

Sleep is the big one. Obstructive sleep apnoea can cause loud snoring, breathing pauses, waking choking or gasping, morning headaches, poor concentration and daytime tiredness.⁷ Blood tests may be normal because the problem is overnight breathing and sleep fragmentation. If someone tells you that you stop breathing in sleep, that is more important than another supplement panel.

ME/CFS and long COVID can also be missed by routine blood tests. NICE says ME/CFS should be suspected when symptoms including debilitating fatigue, post-exertional malaise, unrefreshing sleep, cognitive difficulties and other symptoms are present and persist for the specified time period.² Post-exertional malaise means symptoms worsen after physical, cognitive or emotional effort, often delayed. Pushing harder can make this pattern worse.

Medication effects matter. Antihistamines, some antidepressants, beta blockers, opioids, sedatives, some blood pressure medicines and alcohol can all contribute to fatigue. Pain, poor sleep, caring responsibilities, shift work, grief, depression, anxiety and burnout can also produce real physical exhaustion. These are not "just in your head"; they are physiology plus life load.

Use the health library to understand conditions that can cause fatigue, Start Here to build a symptom timeline, insights to judge claims about private testing, and the stack builder to list medicines and supplements before your appointment.

How to prepare for the appointment

Bring a one-page fatigue summary. Include when it started, whether it is constant or episodic, sleep quality, snoring, post-exertional crashes, mood, pain, weight change, appetite, thirst, urination, bowel change, periods, infections, medicines, alcohol, training load, work pattern and what you cannot do now that you could do before.

Ask for tests in pairs: the test and the reason. For example: "I have heavy periods and breathlessness on stairs. Can we check full blood count and ferritin?" Or: "I have fatigue, bloating and iron deficiency. Should we do coeliac serology before I change my diet?" Or: "My partner says I stop breathing in sleep. Can we assess sleep apnoea?" This is harder to dismiss than "I want a full panel."

If the GP refuses a test, ask calmly: "What would make this test indicated?", "What are we ruling out with the tests we are doing?", "What is the review date?", and "What symptoms should make me seek urgent help?" A good plan may be watchful waiting, but it should not be vague.

When tiredness is urgent

Seek urgent medical help if fatigue is accompanied by chest pain, severe breathlessness, fainting, confusion, one-sided weakness, severe headache, black stools, vomiting blood, new jaundice, very fast or irregular heartbeat, high fever, dehydration, suicidal thoughts, or sudden severe weakness. Use NHS 111 when you need urgent advice and are not sure what to do, and 999 for life-threatening symptoms.¹⁰

Book a GP review promptly if fatigue is new and persistent, worsening, stopping normal activities, associated with weight loss, night sweats, unexplained pain, swollen glands, blood loss, pregnancy, heavy periods, recurrent infections, neurological symptoms, or a clear post-viral decline. If initial tests are normal but function remains poor, ask for a follow-up plan rather than accepting "nothing showed up" as the final answer.

This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.