Why am I so tired all the time?

You sleep eight hours and wake up feeling you've had four. By mid-afternoon you're running on coffee, and the weekend goes on recovering rather than living. Everyone tells you it's just being busy.

Feeling tired all the time is one of the most common reasons people see a GP, and often the cause is identifiable and treatable. Everyone has weeks of running on empty, usually explained by poor sleep, overwork or stress. The kind worth investigating doesn't improve with rest, lasts more than a few weeks, and starts affecting your concentration, your work or your enjoyment of normal things.

If you'd rather skip the reading, a private GP consultation (£150, in person) is the usual first step. A doctor's history and examination decide which blood tests are worth doing, and many routine results are back within one working day.

When should tiredness be checked promptly?

See a doctor soon if your tiredness comes with any of these:

• Unexplained weight loss
• Night sweats
• Persistent fever
• Blood in your urine or stool
• New lumps or swollen lymph nodes
• Severe muscle weakness, rather than just feeling tired
• Persistent low mood or thoughts of self-harm

These are worth investigating to rule out a serious underlying condition. Call 0207 499 1991 for a same-day appointment.

Why am I always tired? Common medical causes

Iron deficiency. The single most common medical cause of fatigue, especially in women of reproductive age. Heavy periods, pregnancy and a diet low in red meat or other iron-rich foods all deplete iron stores. Here's the thing most people don't know: you can be iron-deficient without being anaemic. Your haemoglobin may be normal while your ferritin, your stored iron, is low. Symptoms include tiredness, brain fog, breathlessness on exertion, pale skin and brittle nails, and treatment with iron is checked with repeat blood tests. If a doctor has already decided you need iron studies, the iron status profile at Blood London is the matching single profile.

An underactive thyroid. Hypothyroidism slows your metabolism, causing tiredness, weight gain, feeling cold, constipation and dry skin. It affects about 2% of the UK population and is more common in women over 40. A blood test for TSH (thyroid-stimulating hormone) and free T4 confirms it, and treatment with levothyroxine is lifelong but straightforward. Our guide to thyroid symptoms and testing explains which thyroid profile to book.

Low vitamin D. Widespread in the UK, particularly from October to March, when sunlight is too weak for the skin to make enough. Deficiency causes fatigue, muscle aches and low mood. Government advice, given by the NHS and the Department of Health and Social Care, is that all adults consider 10 micrograms (400 IU) a day in autumn and winter. A blood test confirms your level, and if it's very low your GP may prescribe a loading dose before moving to a maintenance dose.

Low vitamin B12. More common in vegetarians, vegans and older adults. The symptoms overlap with iron deficiency: fatigue, weakness, tingling in the hands or feet and poor concentration. Pernicious anaemia, an autoimmune condition that stops B12 being absorbed, is the most serious form and is treated with B12 injections rather than tablets.

Diabetes. Both type 1 and type 2 diabetes cause fatigue, often as one of the first symptoms, because the body can't use glucose properly for fuel. Thirst, passing more urine and unexplained weight change alongside tiredness should prompt an HbA1c blood test.

Coeliac disease. An autoimmune reaction to gluten that affects about 1 in 100 people in the UK, many of them undiagnosed. Tiredness is common, often with bloating, loose stools or iron deficiency that keeps coming back. The blood test is only reliable while you're still eating gluten, so don't cut it out before you're tested.

Depression and anxiety. Mental health conditions cause real physical fatigue. Depression in particular brings a bone-deep tiredness that sleep doesn't fix, often with low mood, loss of interest, appetite changes or poor sleep. It isn't "all in your head": the fatigue of depression has a neurochemical basis and is entirely real.

Sleep problems. Seven hours in bed doesn't always mean seven hours of good sleep. Sleep apnoea, where breathing stops and starts through the night, leaves people exhausted without knowing why. If you nod off during the day rather than just feeling drained, read our guide to falling asleep during the day.

After a virus, including long COVID. Flu, glandular fever and COVID-19 can leave fatigue behind for weeks. NICE calls symptoms lasting 4 to 12 weeks after COVID-19 ongoing symptomatic COVID-19, and symptoms beyond 12 weeks post-COVID-19 syndrome, when nothing else explains them.

Medicines. Some blood pressure tablets, antihistamines, antidepressants and strong painkillers cause tiredness. If it started after a new medicine or a dose change, mention it to your GP rather than stopping the medicine yourself.

Why am I so tired as a woman?

Women are more likely to be short of iron, because of periods and pregnancy, and more likely to develop an underactive thyroid after 40. Perimenopause is the other big one. In the years before periods stop (the average age of menopause in the UK is 51), changing hormone levels can bring tiredness, broken sleep, night sweats, low mood and brain fog, often well before anyone mentions the menopause.

You don't need a blood test to start that conversation. Our article on seeing a doctor about perimenopause explains why, and a menopause GP consultation with Dr Penny Sheehan is £150 for 30 minutes, on Mondays and Wednesdays.

Tired in your 30s or 40s, or suddenly tired lately?

In your 30s and 40s, work, young children and short nights explain a lot, but they also hide things. Iron deficiency, thyroid problems, perimenopause and early type 2 diabetes all tend to appear in these decades, and they're easy to put down to a busy life.

A sudden change matters more than a slow one. If you've gone from your normal energy to exhausted over a few weeks, think about what changed: a virus, a new medicine, a stressful event, heavier periods. If none of those fits, or the tiredness comes with any of the warning signs above, book sooner rather than waiting it out.

My blood tests are normal, so why am I still tired?

A basic blood count often leaves out ferritin, B12, folate, vitamin D and a coeliac screen, so ask what was actually tested. Some causes don't show up on bloods at all: sleep apnoea, depression and the after-effects of a virus are found through the history, not the lab. And if exhausting fatigue that gets worse after activity has lasted months, a doctor will consider ME/CFS once other causes have been looked for.

Blood tests commonly used to investigate tiredness.

You can book single profiles through Blood London if you already know which test you need. A GP consultation (£150) is the better start if you don't, and the fatigue assessment (£1,499) covers every test in this table plus hormones, a brain MRI and an ECG.

Everyday habits that drain your energy

It's worth looking at the basics alongside any medical cause. Poor sleep from screens, irregular hours or sleep apnoea is a major contributor. Alcohol disrupts restorative sleep even in moderate amounts, and caffeine after midday affects sleep more than most people realise.

Being inactive, oddly, causes fatigue rather than preventing it. Regular moderate exercise consistently improves energy levels in clinical trials, and if you're sedentary and tired, 20 to 30 minutes of walking a day is one of the best-evidenced places to start.

How we can help

An assessment or a blood test on its own? If a doctor has already told you which test you need, a single test or profile can be enough. If you're not sure why you're tired, start with an assessment: the doctor takes a history and examines you first, so the tests are chosen for a reason rather than picked from a list.

A private GP consultation (£150, in person) is the usual first step. For tiredness that has lasted several weeks without an obvious cause, including after a viral illness, our fatigue assessment (£1,499) includes a doctor's examination, a brain MRI, a 12-lead ECG and a targeted blood panel covering full blood count and ESR, iron and ferritin, thyroid function, vitamin B12, folate and vitamin D, liver and kidney function, hormones, Epstein-Barr virus antibodies and coeliac screening.

Blood results take 3 to 5 working days and the MRI report 7 to 10 working days. Your GP reviews everything with you in a follow-up consultation, explains what has been found and starts treatment if needed, whether that's a prescription, a supplement plan or an onward referral. For a broader check that combines fatigue markers with heart, metabolic and organ screening, look at our Full Body MOT health check.