Why am I always tired? A GP's guide to the real…

Written by Dr James Coleman, GP and founder of Brooksby Medical.
Medically reviewed: May 2026 · Next review due: May 2027.

“Why am I always tired?” is one of the most common questions I hear in clinic, and one of the hardest to answer in a ten-minute appointment.

Tiredness can mean many different things. For one person, it may be the result of a stressful month, poor sleep, or emotional strain. For another, it may be the first clue to iron deficiency, an underactive thyroid, diabetes, sleep apnoea, perimenopause, or a vitamin deficiency.

Working out which one applies to you usually takes a proper look at your symptoms, your lifestyle, your sleep, your medical history, and sometimes a blood test.

This guide explains the most common causes of fatigue I see in general practice, what blood tests can and cannot show, and the warning signs that mean you should seek medical help promptly rather than ordering a private blood test.

Where I mention thresholds or guidelines, I’ve used UK guidance because that is what your own GP will usually be working from.

The short answer: common causes of feeling tired all the time include poor sleep, chronic stress, iron deficiency, thyroid problems, vitamin D or B12 deficiency, diabetes or pre-diabetes, perimenopause, testosterone deficiency in men with relevant sexual symptoms, sleep apnoea, depression, anxiety, and post-viral fatigue. If your tiredness is persistent, getting worse, or comes with red-flag symptoms such as unexplained weight loss, night sweats, breathlessness, chest pain, neurological symptoms, new lumps, or bleeding, it should be assessed clinically rather than self-investigated.

Fatigue is a symptom, not a diagnosis

The first thing worth understanding is that fatigue is not a diagnosis. It is a description of how you feel.

Recognising fatigue as a clinical complaint is exactly what the NICE Clinical Knowledge Summary on tiredness in adults frames it as: a non-specific symptom that needs a structured workup rather than a single test. In medicine, we often think about tiredness in broad timeframes. Acute fatigue lasts less than four weeks and usually has an obvious trigger: a viral illness, a few bad nights of sleep, a demanding stretch at work, emotional stress, or overtraining. This kind of tiredness often improves by itself.

Fatigue that lasts more than four weeks, is not improving, or is starting to affect your work, relationships, exercise, or day-to-day life is worth reviewing properly.

If fatigue lasts three months or more and comes with post-exertional malaise — symptoms worsening 12 to 48 hours after physical or mental effort — along with unrefreshing sleep and cognitive symptoms, then myalgic encephalomyelitis / chronic fatigue syndrome (ME/CFS) becomes part of the differential and should be assessed against NICE NG206 criteria.

In practice, many patients with persistent tiredness have one or more treatable contributors, but fatigue is often multifactorial. The aim is not to assume one cause. It is to look carefully for the common medical, lifestyle, sleep, and mental health factors that may be driving it.

The common causes I look for in clinic

These are not ranked by seriousness. They are grouped around the causes I most commonly look for when someone comes to clinic feeling tired all the time.

1. Iron deficiency and low ferritin

Iron deficiency is one of the most common treatable causes of fatigue I diagnose, especially in women of reproductive age.

People often miss it because iron stores can fall long before a full blood count shows anaemia. The marker that matters most here is ferritin, which reflects stored iron.

According to NICE Clinical Knowledge Summaries, ferritin below 30 micrograms/L is generally consistent with iron deficiency, even if haemoglobin is still normal. Ferritin can also be falsely raised by inflammation, infection, or liver disease, so it should be interpreted alongside CRP and the wider clinical picture.

Symptoms of iron deficiency can include:

tiredness
shortness of breath on exertion
restless legs at night
brittle nails
hair shedding
reduced exercise tolerance
unusual non-food cravings, known as pica

Heavy periods are a common cause in younger women. Other causes include pregnancy, recent childbirth, vegetarian or vegan diets, coeliac disease, inflammatory bowel disease, poor dietary intake, and chronic blood loss from the gut.

Iron stores can also be depleted after childbirth, especially after significant blood loss, antenatal anaemia, or heavy ongoing periods. NICE considers haemoglobin below 100 g/L in the postpartum period to be anaemic, but symptoms and ferritin still matter because iron deficiency can persist even after haemoglobin improves.

2. An underactive thyroid

Hypothyroidism is another important cause I would want to rule out.

Your thyroid helps set the metabolic tempo for your body. When it is underactive, everything can feel slower: energy, mood, bowels, skin, hair, and temperature regulation.

The classic pattern is tiredness alongside:

weight gain
feeling cold when others do not
dry skin
constipation
low mood
heavy periods
brain fog
slow pulse
hair thinning

The first-line blood test is TSH, usually followed by free T4 if TSH is outside the reference range.

Per the NICE Clinical Knowledge Summary on hypothyroidism, overt hypothyroidism is typically a raised TSH with a low free T4 and usually warrants treatment. Subclinical hypothyroidism — a mildly raised TSH with a normal free T4 — is more nuanced. It often needs repeat testing and clinical interpretation, particularly if symptoms are vague.

A single mildly raised TSH does not automatically mean “you have hypothyroidism”. It needs context.

Hypothyroidism is more common in women, after pregnancy, and with increasing age, particularly over 40.

3. Vitamin D deficiency

Vitamin D deficiency is common in the UK, especially through autumn and winter when sunlight is not strong enough for reliable vitamin D production in the skin.

The NHS recommends that adults consider taking a daily 10 microgram vitamin D supplement from October to March. Current public health advice is supplementation rather than routine testing for everyone.

That said, vitamin D is worth checking in some people, particularly if they have:

darker skin
limited sun exposure
clothing that covers most of the skin outdoors
persistent muscle aches
bone pain
muscle weakness
unexplained fatigue

According to the NICE Clinical Knowledge Summary on vitamin D deficiency in adults, symptoms can be vague and may include tiredness, muscle aches, muscle weakness, and bone pain. Low mood and recurrent infections are sometimes discussed in this context, but they are less specific and should not be assumed to be caused by vitamin D deficiency alone.

4. B12 and folate deficiency

Vitamin B12 and folate are essential for healthy red blood cells and normal nervous system function.

Deficiency in either can cause fatigue. B12 deficiency is particularly important because it can also cause neurological symptoms, including:

pins and needles
numbness
poor balance
brain fog
memory problems
tongue soreness
mood change

NICE NG239 makes clear that B12 deficiency can occur even without anaemia or macrocytosis, so symptoms and risk factors matter as much as the full blood count.

Higher-risk groups include:

vegans and vegetarians
people over 60
people with pernicious anaemia
people with coeliac disease or inflammatory bowel disease
people who have had gastric or bowel surgery
people taking long-term metformin
people taking long-term proton pump inhibitors such as omeprazole or lansoprazole

One increasingly important cause to flag is recreational nitrous oxide use. Nitrous oxide can inactivate B12 and produce neurological symptoms even when serum B12 levels look normal. If nitrous oxide-related B12 deficiency is suspected, NICE recommends plasma homocysteine or serum methylmalonic acid rather than relying on serum B12 alone.

Either total B12 or active B12 can be used as an initial test. Active B12 may be helpful in some borderline cases, but results still need interpreting alongside symptoms and risk factors.

5. Blood sugar problems

Pre-diabetes and type 2 diabetes are common enough in the UK that I think about blood sugar in any adult with persistent tiredness, especially if they are over 40, carry weight around the middle, have a family history of diabetes, or have a history of gestational diabetes.

The relevant test is HbA1c, which reflects average blood sugar over the previous two to three months.

Per NICE PH38 and NICE NG28, an HbA1c of 42–47 mmol/mol is treated as a high-risk or pre-diabetes range. An HbA1c of 48 mmol/mol or above is in the diabetes range and should be interpreted in context, with confirmation if the person has no classic symptoms.

Fatigue can be an early symptom of diabetes, often alongside:

increased thirst
passing urine more often
recurrent infections
blurred vision
slow wound healing
weight change

6. Perimenopause, low testosterone, and hormonal shifts

Hormonal causes need careful wording because not every hormone-related symptom needs a hormone blood test.

In women in their late thirties, forties, and early fifties, perimenopause is one of the most commonly missed contributors to tiredness I see. Falling and fluctuating oestrogen can disrupt sleep, mood, concentration, and energy in ways that often look like depression, anxiety, or burnout.

Common symptoms include:

disrupted periods
heavier or more irregular bleeding
hot flushes
night sweats
poor sleep
brain fog
irritability
new anxiety
reduced libido
joint aches
fatigue

Crucially, NICE QS143 is clear that in women aged 45 and over with typical symptoms, perimenopause is diagnosed clinically from the history, not from routine FSH or oestradiol blood tests.

Hormone tests can occasionally help in women under 45 or where the picture is atypical, but they are not the standard diagnostic route. What blood tests can help with at this life stage is excluding common mimics or contributors, including iron deficiency, thyroid disease, B12 deficiency, vitamin D deficiency, and pre-diabetes.

In men, low testosterone can contribute to fatigue, low mood, reduced motivation, reduced libido, erectile dysfunction, and loss of morning erections. Testosterone testing is most useful when fatigue occurs alongside sexual symptoms. Fatigue alone is rarely enough to diagnose testosterone deficiency.

Per BSSM guidance, testosterone deficiency is usually considered when total testosterone is below 12 nmol/L on two morning samples, alongside relevant symptoms.

7. Sleep debt and poor sleep quality

Sometimes the answer is the obvious one: not enough sleep.

The NHS recommends seven to nine hours of sleep per night for most adults. A surprising number of people are running on six hours and assuming that should be enough.

Sleep quality matters as much as sleep quantity. If you are in bed for long enough but still waking unrefreshed, it is worth thinking about sleep disruption.

One important cause is obstructive sleep apnoea. This is more likely if you:

snore heavily
wake with a dry mouth
wake with morning headaches
feel exhausted despite a full night in bed
have witnessed pauses in breathing during sleep
wake choking or gasping
feel sleepy during the day
have high blood pressure

Sleep apnoea is not just a tiredness problem. Untreated, it is associated with cardiovascular risk and road traffic risk due to daytime sleepiness. It is often under-recognised and worth raising with your GP.

8. Chronic stress, anxiety, and low mood

This is the cause no blood test will find.

Chronic stress, generalised anxiety, depression, and burnout can all cause profound fatigue. They disrupt sleep, drain motivation, reduce concentration, and make normal daily tasks feel disproportionately effortful.

This is not a “diagnosis of exclusion”. It is a real, common, and treatable cause of tiredness.

It is also common for mental health and physical contributors to coexist. Someone can have iron deficiency and anxiety. Someone can be perimenopausal and burnt out. Someone can have poor sleep and borderline thyroid results.

If you have been low for weeks, have lost interest in things you usually enjoy, feel constantly on edge, or are struggling to function, please do not wait for blood tests before raising it with your GP.

Why am I always tired and have no energy? Women

This is one of the most-searched fatigue questions in the UK, and the answer often changes with age.

In women under 40, iron deficiency is one of the leading treatable causes. Heavy or prolonged periods can deplete iron stores faster than diet can replace them, especially on a vegetarian, vegan, or low-meat diet. Pregnancy, breastfeeding, and the months after childbirth can also increase iron demand.

Between 40 and 55, perimenopause becomes one of the most important causes to consider. Hormonal change can disrupt sleep architecture, meaning seven or eight hours in bed may not feel as restorative as it used to. Fatigue, foggy thinking, sleep disruption, new anxiety, night sweats, and a sense of “running on a lower setting” are all common.

Women with autoimmune thyroid disease often present in this same broad age range, which is why a thyroid check belongs in many fatigue work-ups.

If you are in this age range and persistently tired, blood tests can be useful, but mainly to look for common mimics or contributors such as iron deficiency, thyroid disease, B12 deficiency, vitamin D deficiency, and pre-diabetes — not to diagnose perimenopause itself in women aged 45 and over with typical symptoms.

Why do I feel tired after eating?

Some post-meal drowsiness is normal. After eating, blood flow to the digestive system increases and the parasympathetic nervous system shifts the body into a more relaxed state.

That is physiology, not pathology.

What is less normal is significant, persistent fatigue after meals, especially if it happens repeatedly after carbohydrate-heavy meals or comes with shakiness, sweating, hunger, palpitations, or light-headedness.

This pattern can occasionally reflect post-meal glucose swings, including reactive hypoglycaemia, but that is not the most common explanation. HbA1c is a sensible first-line screen for diabetes risk, but persistent symptoms may need clinical assessment rather than assuming a glucose problem.

Iron deficiency, thyroid disease, poor sleep, and stress can all make post-meal fatigue feel worse.

Tired and getting headaches?

Headaches plus fatigue is a common combination, and the causes often overlap with the list above.

Common possibilities include:

iron deficiency anaemia
dehydration
poor sleep
stress or tension-type headache
migraine
medication overuse headache
viral illness
thyroid disease
sleep apnoea

Vitamin D deficiency can contribute to tiredness and muscle symptoms, though headaches are non-specific and often have other causes.

What would worry me is a new headache pattern that is different from your usual headaches, headaches that wake you from sleep, headaches with visual disturbance, fever, weight loss, jaw pain, confusion, weakness, numbness, or a sudden thunderclap onset.

Those need urgent medical assessment.

Tired and short of breath: when to worry

Fatigue with breathlessness is more concerning than fatigue alone.

It can be straightforward, such as deconditioning or anaemia. But it can also reflect more serious problems, including heart failure, lung disease, infection, pulmonary embolism, or significant anaemia.

The questions I would ask are:

Has the breathlessness come on suddenly over hours or days?
Is it getting worse quickly?
Does it worsen when lying flat?
Does it wake you at night?
Is there chest pain?
Is there ankle swelling?
Is there a new cough?
Is there coughing up blood?
Is there a racing or irregular heartbeat?

If any of those apply, do not book a routine appointment in several weeks. Contact your GP, NHS 111, or emergency services depending on severity.

What about post-viral fatigue and long COVID?

Post-viral fatigue can happen after many infections, including glandular fever, influenza, and COVID-19.

Most people gradually recover over weeks to months. A minority do not, and this is where post-COVID syndrome or ME/CFS may become part of the picture.

The hallmark feature to ask about is post-exertional malaise. This means symptoms worsen after physical, mental, or emotional effort, often 12 to 48 hours later, and the crash feels out of proportion to the activity.

For example, someone may push through a normal day and then feel significantly worse the next morning.

NICE NG206 sets out the diagnostic criteria for ME/CFS and recommends energy management rather than fixed incremental graded exercise therapy.

A blood test will not diagnose post-viral fatigue, long COVID, or ME/CFS. There is no single biomarker. What blood tests can do is help rule out other treatable causes before making that diagnosis.

What vitamin deficiency causes fatigue?

Several nutritional deficiencies can contribute to fatigue, and more than one can occur at the same time.

The main ones I look for are:

Iron — technically a mineral, but often grouped with this question. Low iron stores can cause fatigue with or without anaemia.
Vitamin D — common in the UK, especially in winter and in people with darker skin or limited sun exposure.
Vitamin B12 — especially relevant in vegans, vegetarians, older adults, people taking metformin or acid-suppressing medication, and people with gut conditions.
Folate — important for red blood cell production and often assessed alongside B12.

A targeted blood panel checking these can identify many of the common nutritional contributors to tiredness. Magnesium deficiency is occasionally relevant, but it is much less common than iron, vitamin D, B12, and folate problems.

When fatigue is a red flag

Most fatigue is not caused by something sinister. But some patterns should prompt a same-week GP appointment, same-day assessment, or emergency care depending on severity.

See your GP urgently if fatigue is accompanied by:

unexplained weight loss
night sweats
persistent fever
a new lump or swelling
coughing up blood
blood in your stool or urine
a persistent change in bowel habit
new or severe shortness of breath
chest pain
new neurological symptoms, such as weakness, numbness, slurred speech, facial droop, or sudden visual disturbance

These combinations can occasionally signal serious underlying disease, including cancer, heart disease, lung disease, infection, or neurological illness. The combination of fatigue with any of the red-flag features above is exactly what NICE NG12 (Suspected cancer: recognition and referral) expects to trigger an urgent assessment.

A private blood test is not the right starting point if any of these apply. Your GP, NHS 111, or A&E may be more appropriate depending on the symptom and severity.

What blood tests actually help?

For most adults with persistent unexplained fatigue and no red flags, a structured blood panel can help identify common treatable contributors.

The exact tests should depend on symptoms, age, sex, medications, menstrual history, medical background, and risk factors. Commonly useful tests include:

Full blood count and ferritin — for anaemia, iron deficiency, and other haematological causes
TSH and free T4 — for thyroid disease
Vitamin D — where there is a clinical reason to test rather than supplement empirically
B12 and folate — for nutritional causes affecting red cells and the nervous system
HbA1c — for pre-diabetes and diabetes
Liver and kidney function — because chronic disease can cause fatigue
Calcium and magnesium — less common, but occasionally relevant
CRP — a marker of inflammation and useful when interpreting ferritin
Coeliac antibodies — because coeliac disease can cause fatigue and is under-diagnosed
Cortisol — usually as an 8–9 am serum cortisol test, and only where clinically appropriate, mainly if symptoms suggest adrenal insufficiency (NICE NG243), such as unexplained weight loss, postural dizziness, salt craving, nausea, vomiting, low sodium, or recent steroid withdrawal

Cortisol is not a general “stress” or “burnout” test.

Tests that do not earn their place are those with no validated link to fatigue. Most commercial “food intolerance” tests fall into this category. Both the NHS and the British Dietetic Association advise against IgG-based food intolerance testing because the evidence does not support it.

Hair mineral analysis sits in the same group, as do broad direct-to-consumer hormone panels in healthy adults under 40 with no symptoms beyond tiredness.

The aim of a good fatigue panel is not to throw markers at the wall. It is to test the things that can genuinely change what you do next.

Frequently asked questions

Why am I always tired even after sleeping?

If you are getting seven to nine hours in bed and still waking unrefreshed, the issue is often sleep quality rather than sleep quantity.

Common causes include obstructive sleep apnoea, iron deficiency, vitamin D deficiency, an underactive thyroid, perimenopause, depression, anxiety, chronic stress, or post-viral fatigue.

If unrefreshing sleep comes with symptoms that worsen after exertion, ME/CFS may also need to be considered.

What blood tests should I get for fatigue?

Commonly useful tests in a fatigue work-up include full blood count and ferritin, TSH with free T4 if abnormal, HbA1c, B12 and folate, vitamin D where clinically appropriate, liver and kidney function, calcium, magnesium, CRP, and coeliac antibodies.

Cortisol is appropriate only if symptoms suggest adrenal insufficiency. Hormone tests are not the standard route to diagnose perimenopause in women aged 45 and over.

The exact mix should reflect your symptoms, age, sex, medications, menstrual history, and risk factors, ideally with clinical input.

Can low ferritin cause tiredness without anaemia?

Yes. Ferritin reflects iron stores, and these can fall before haemoglobin drops into the anaemic range.

NICE considers ferritin below 30 micrograms/L consistent with iron deficiency, even if the full blood count is normal.

Symptoms can include tiredness, breathlessness on exertion, restless legs, hair shedding, and reduced exercise tolerance. Ferritin can be falsely raised by inflammation, so it should be interpreted alongside CRP and the clinical picture.

Can menopause cause tiredness?

Yes. Perimenopause can cause tiredness through disrupted sleep, night sweats, hot flushes, mood change, brain fog, and hormonal fluctuation.

In women aged 45 and over with typical symptoms, NICE recommends diagnosing perimenopause from the history rather than from routine FSH or oestradiol blood tests.

Blood tests in this age group are usually more useful for ruling out treatable mimics such as iron deficiency, thyroid disease, B12 deficiency, vitamin D deficiency, or diabetes risk.

Can stress cause extreme tiredness?

Yes. Chronic stress, anxiety, low mood, and burnout can all cause profound fatigue.

They often work by disrupting sleep, increasing physical tension, reducing motivation, and making normal daily tasks feel more effortful.

This is not just “in your head”. Stress-related fatigue is real and treatable, and it can coexist with physical causes such as iron deficiency, thyroid disease, or perimenopause.

When should I worry about fatigue?

Seek urgent medical advice if fatigue comes with unexplained weight loss, night sweats, persistent fever, a new lump, blood in your stool or urine, coughing up blood, a persistent change in bowel habit, new severe breathlessness, chest pain, or new neurological symptoms such as weakness, numbness, slurred speech, or sudden visual disturbance.

These combinations need clinical assessment. A private blood test is not the right starting point.

References

National Institute for Health and Care Excellence. Tiredness/fatigue in adults: Clinical Knowledge Summary. cks.nice.org.uk/topics/tiredness-fatigue-in-adults
National Institute for Health and Care Excellence. Anaemia — iron deficiency: Clinical Knowledge Summary. cks.nice.org.uk/topics/anaemia-iron-deficiency
National Institute for Health and Care Excellence. Hypothyroidism: Clinical Knowledge Summary. cks.nice.org.uk/topics/hypothyroidism
National Institute for Health and Care Excellence. Vitamin D deficiency in adults: Clinical Knowledge Summary. cks.nice.org.uk/topics/vitamin-d-deficiency-in-adults
National Institute for Health and Care Excellence. NG28: Type 2 diabetes in adults: management. nice.org.uk/guidance/ng28
National Institute for Health and Care Excellence. PH38: Type 2 diabetes: prevention in people at high risk. nice.org.uk/guidance/ph38
National Institute for Health and Care Excellence. NG206: Myalgic encephalomyelitis/chronic fatigue syndrome: diagnosis and management. nice.org.uk/guidance/ng206
National Institute for Health and Care Excellence. NG239: Vitamin B12 deficiency in over 16s: diagnosis and management. nice.org.uk/guidance/ng239
National Institute for Health and Care Excellence. NG243: Adrenal insufficiency: identification and management. nice.org.uk/guidance/ng243
National Institute for Health and Care Excellence. QS143: Menopause. nice.org.uk/guidance/qs143
National Institute for Health and Care Excellence. NG12: Suspected cancer: recognition and referral. nice.org.uk/guidance/ng12
NHS. Vitamin D. nhs.uk/conditions/vitamins-and-minerals/vitamin-d
NHS. How to get to sleep. nhs.uk/live-well/sleep-and-tiredness/how-to-get-to-sleep
NHS. Food intolerance. nhs.uk/conditions/food-intolerance
British Dietetic Association. Food allergy and food intolerance testing. bda.uk.com/resource/food-allergy-intolerance-testing
Hackett G, et al. The British Society for Sexual Medicine Guidelines on Male Adult Testosterone Deficiency, with Statements for Practice. Int J Impot Res. 2023. pmc.ncbi.nlm.nih.gov/articles/PMC10307648

If you have had persistent fatigue for more than a few weeks, have no red-flag symptoms, and want a structured, GP-led assessment to look at the common contributors covered in this article, our GP-led fatigue assessment combines a 50-biomarker blood test with a one-to-one consultation with Dr Coleman to look at both blood-test and non-blood-test causes of tiredness.

Written by Dr James Coleman, GP and founder of Brooksby Medical. Dr Coleman is a practising General Practitioner who founded Brooksby Medical to give patients timely access to private blood testing with GP interpretation, while helping them understand when NHS or in-person care is more appropriate.

Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, medical history, and clinical picture. If you have concerns about your health, please consult your GP.

Why am I always tired? A GP's guide to the real causes of fatigue and what to do about it