Why Am I Always Tired? A Doctor’s Guide to the Causes of Persistent Fatigue
Tiredness is the single most common thing we hear at the clinic. Most of the time it is not a mystery and not something you simply have to live with. Persistent fatigue is a signpost, and this is the map a doctor follows to find what it is pointing at.
- Ongoing tiredness is a symptom with a real medical differential, not a character flaw.
- Most causes are common and checkable: sleep, iron, thyroid, vitamins, blood sugar, hormones and mood.
- The goal of testing is to find the cause. Treatment follows the cause, and only when there is one to treat.
Tired, or fatigued?
There is a difference between being tired and being fatigued. Tired lifts after a good night of sleep and a lighter week. Fatigue is the heavier kind that a weekend does not fix, and it is the kind worth investigating. A fair first question is simply whether you are getting enough sleep: adults need seven or more hours a night, and a large share of people quietly run under that for years.1
If the sleep is genuinely there and the tiredness stays, that is the signal to look further. Below is what we look at, roughly in the order it tends to matter.
The eight things we check
No single test explains every case, so a good work-up looks across several systems at once. Here are the usual suspects, what each tends to feel like, and how it is confirmed.
Sleep and sleep apnoea
How it shows upWaking unrefreshed, heavy afternoons, loud snoring or a partner noticing you stop breathing.
How we checkA sleep and lifestyle history, and a referral for a sleep study if apnoea is likely.
Low iron
How it shows upFatigue with breathlessness on stairs, cold hands, or heavy periods. It can start before anaemia shows.
How we checkA full blood count plus ferritin and transferrin saturation.
Thyroid
How it shows upTiredness with weight gain, feeling the cold, dry skin, or low mood.
How we checkA simple blood test: TSH, and free T4 if needed.
Vitamin B12, D and folate
How it shows upPersistent tiredness, sometimes with pins and needles (B12) or aching (vitamin D).
How we checkSerum B12, folate and 25-hydroxy vitamin D.
Blood sugar
How it shows upEnergy crashes after meals, thirst, or a family history of diabetes.
How we checkHbA1c and a fasting glucose.
Hormones
How it shows upIn women, tiredness and broken sleep around perimenopause. In men, low energy that has crept up over time.
How we checkTargeted hormone bloods, read alongside your symptoms.
Mood and stress
How it shows upFatigue that comes with low mood, worry, or simply too much on. Very common, and easy to dismiss.
How we checkAn honest conversation, and a short validated questionnaire.
After an infection
How it shows upTiredness that lingers for weeks or months after a virus, including COVID.
How we checkA history, an exam, and bloods to rule out other causes first.
Roughly one in eight adults has an underactive thyroid or a milder version of it, more often women and more often with age. It is one of the most treatable causes of fatigue, and it is a simple blood test.5
How a doctor narrows it down
Good medicine is not a scattergun of every test that exists. It is a sequence: listen, look, confirm, then act on what the results actually show.
- 1 The story When it started, how you sleep, your mood, your cycle, medications, alcohol and caffeine. The history points to where to look.
- 2 The right tests A focused panel: full blood count, ferritin, thyroid, B12, folate, vitamin D and HbA1c, plus anything your story adds.
- 3 Interpretation A number inside the "normal" range is not always right for you. Results are read together, and against how you feel.
- 4 A plan Correct what is low, treat what is found, and change what is driving it. No finding, no treatment for its own sake.
Two examples of why this order matters. In menstruating women, low iron can cause real fatigue before anaemia ever shows on a standard blood count. In one randomised trial, women with low ferritin and unexplained tiredness who took iron saw their fatigue fall by almost half, well beyond the placebo group.3,4 And vitamin D is similar: in a placebo-controlled trial, correcting a genuine deficiency improved self-reported fatigue.7 The lesson is not "take iron and vitamin D." It is "measure first, then replace what is actually missing."
Sleep deserves its own mention. Obstructive sleep apnoea, where breathing repeatedly pauses in the night, is thought to affect close to a billion adults worldwide, and the large majority have never been diagnosed.2 If you snore heavily and wake unrefreshed, that is worth chasing down rather than blaming on age or a busy calendar.
"It's my adrenals."
What people mean: that chronic stress has "burned out" the adrenal glands and left them unable to make enough cortisol, causing the tiredness.
What the evidence says: a systematic review of 58 studies found no support for "adrenal fatigue" as a real condition.8 Genuine adrenal disease does exist, and it is diagnosed with proper testing, not a saliva kit. Chasing a label that does not exist is time not spent finding the cause that does.
"A drip will sort my energy."
An intravenous vitamin can restore a proven deficiency quickly, and that is a fair use of it. But if your levels are already normal, a drip has nothing to correct. We would rather test first and treat what is real than sell you a top-up your body does not need.
The foundations that fix a lot of it
Before and alongside any test, a handful of ordinary things move the needle more than most people expect. They also make your results cleaner to read.
- Protect your sleep window. A consistent time to bed and to wake, and seven hours or more, does more than any supplement.1
- Move a little, most days. It sounds backwards, but gentle regular exercise reduces fatigue. In one trial of sedentary adults with persistent tiredness, low-intensity activity cut fatigue by around 65 percent.9
- Eat to steady your blood sugar. Regular meals with protein and fibre soften the mid-afternoon crash.
- Mind caffeine and alcohol timing. Both wreck sleep quality even when they do not stop you falling asleep.
- Get daylight early. Morning light helps set the body clock that governs your energy.
Tiredness with any of these deserves prompt review
- Unexplained weight loss, night sweats or a persistent fever
- Breathlessness, chest pain, or a pounding or irregular heartbeat
- Fatigue that is severe, came on suddenly, or is getting steadily worse
- Tiredness that has not lifted after a few weeks of good sleep and sensible habits
This article is general education, not a diagnosis. If something here rings true, see a doctor who can assess you properly.11
A quick reference
If you like it on one line, here is the short map of the common causes, what they tend to feel like, and the test that finds them.
| Area | Common signs | Typical test |
|---|---|---|
| Sleep and sleep apnoea | Unrefreshing sleep, snoring, daytime sleepiness | Sleep history, sleep study if indicated |
| Low iron | Breathlessness, pallor, heavy periods | Full blood count, ferritin, transferrin saturation |
| Underactive thyroid | Cold intolerance, weight gain, dry skin | TSH, free T4 |
| B12 or folate | Tiredness, pins and needles, sore tongue | Serum B12, folate |
| Vitamin D | Low energy, aches, low mood | 25-hydroxy vitamin D |
| Blood sugar | Post-meal crashes, thirst, family history | HbA1c, fasting glucose |
| Mood and stress | Low mood, poor sleep, feeling overwhelmed | Clinical review, validated questionnaire |
How we look at this at Live More
Our approach to persistent tiredness is unglamorous on purpose. We take a proper history, run a focused panel rather than a random one, and then a doctor sits with you and reads the results together, against how you actually feel. If something is low, we correct it. If something needs treating, we treat it. If the answer is sleep, stress or a habit, we say so. It starts with a conversation, and the first thirty-minute consult is free.
Common questions
How long is too long to feel tired?
What blood tests are done for tiredness?
Can a vitamin deficiency really cause fatigue?
Is it normal to feel tired all the time after 40?
Is "adrenal fatigue" a real diagnosis?
Will a vitamin drip fix my energy?
References
- Watson NF, et al. “Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society.” Sleep, 2015;38(6):843–844. View source
- Benjafield AV, et al. “Estimation of the global prevalence and burden of obstructive sleep apnoea: a literature-based analysis.” The Lancet Respiratory Medicine, 2019;7(8):687–698. View source
- Vaucher P, et al. “Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial.” CMAJ, 2012;184(11):1247–1254. View source
- Verdon F, et al. “Iron supplementation for unexplained fatigue in non-anaemic women: double blind randomised placebo controlled trial.” BMJ, 2003;326(7399):1124. View source
- Wiersinga WM, et al. “Hypothyroidism: the difficulty in attributing symptoms to their underlying cause.” Frontiers in Endocrinology, 2023;14:1130661. View source
- Langan RC, Goodbred AJ. “Vitamin B12 Deficiency: Recognition and Management.” American Family Physician, 2017;96(6):384–389. View source
- Nowak A, et al. “Effect of vitamin D3 on self-perceived fatigue: A double-blind randomized placebo-controlled trial.” Medicine (Baltimore), 2016;95(52):e5353. View source
- Cadegiani FA, Kater CE. “Adrenal fatigue does not exist: a systematic review.” BMC Endocrine Disorders, 2016;16(1):48. View source
- Puetz TW, Flowers SS, O’Connor PJ. “A randomized controlled trial of the effect of aerobic exercise training on feelings of energy and fatigue in sedentary young adults with persistent fatigue.” Psychotherapy and Psychosomatics, 2008;77(3):167–174. View source
- Sandler CX, et al. “Long COVID and Post-infective Fatigue Syndrome: A Review.” Open Forum Infectious Diseases, 2021;8(10):ofab440. View source
- National Health Service (NHS). “Tiredness and fatigue.” nhs.uk, accessed 2026. View source