Constant Fatigue and Weakness: Causes and When It's Serious
You wake up already tired. You do ordinary things — and by evening it feels like you've been moving freight. Coffee doesn't help. Many people know this feeling, and most of the time there's an explanation — it just takes some digging to find it. This article covers the most common causes of chronic fatigue, which tests are worth getting, and when weakness is more than just needing better sleep.
Why you're always sleepy — the main causes
Fatigue isn't a diagnosis — it's a symptom. There are dozens of possible causes, and many of them are easily addressed. Here are the most common ones.
Sleep deprivation and irregular sleep patterns. This sounds obvious, but most adults regularly sleep less than 7 hours. Even a modest chronic shortfall — an hour or two less each night — accumulates over weeks into persistent sluggishness, poor concentration, and irritability. Rule this out before ordering tests.
Iron deficiency and anaemia. One of the most common medical causes of fatigue, especially in women of reproductive age. Iron is essential for carrying oxygen to cells. When iron stores are low, muscles and the brain are effectively starved of oxygen. A person can feel deeply fatigued long before anaemia shows up on a blood count — when ferritin is still technically "normal" on the lab report but actually depleted.
Hypothyroidism. The thyroid gland regulates metabolism. When it underperforms, everything slows down: heart rate, thinking, digestion, mood. The classic picture of hypothyroidism is fatigue, feeling cold, weight gain, and apathy. It's common, and it's easy to detect with a TSH test.
Diabetes and prediabetes. Chronic fatigue is one of the earliest signs of impaired glucose metabolism. Cells can't get enough energy, even though blood sugar is elevated. If you feel drained after carbohydrate-heavy meals, it's worth checking fasting glucose and glycated haemoglobin (HbA1c).
Vitamin D and B12 deficiency. Both are critical for nervous system function and cellular energy production. Vitamin D deficiency is common in northern latitudes during autumn and winter. B12 deficiency is especially prevalent in vegetarians, older adults, and people taking metformin.
Chronic stress and overload. Under sustained stress, the adrenal glands pump out cortisol continuously. Over time the system runs down, and a person wakes up already exhausted despite a full night's sleep.
Iron and vitamin deficiency as a cause of fatigue
This deserves a closer look because it's genuinely common and genuinely missed.
Ferritin. This protein stores iron in the body. Ferritin drops first — before haemoglobin falls. Many labs flag "normal" ferritin at 10–15 µg/L, but most specialists consider that optimal ferritin for wellbeing is at least 50–70 µg/L, and for some people over 100. At a ferritin of 15–20 a person can complain of severe fatigue, hair loss, and poor exercise tolerance — and be technically "healthy" on paper.
Vitamin D. The normal range for 25(OH)D is above 30 ng/mL (75 nmol/L). Deficiency is defined as below 20 ng/mL. Research shows that levels below 20 ng/mL are associated with significantly higher rates of chronic fatigue, depressive symptoms, and muscle weakness.
Vitamin B12. Normal range: 200–900 pg/mL (148–664 pmol/L). Deficiency symptoms — fatigue, tingling in the hands and feet, memory problems, low mood — can appear even at the lower end of "normal".
Magnesium. Magnesium is involved in hundreds of biochemical reactions, including cellular energy production. Deficiency presents as muscle cramps, poor sleep, irritability, and fatigue. Serum magnesium doesn't always reflect total body magnesium, so symptoms matter as much as the number.
Fatigue as a symptom of thyroid disease
The thyroid is a small gland at the base of the neck, but when it malfunctions, the effects can touch virtually everything. Hypothyroidism affects around 2–3% of adults, and subclinical hypothyroidism (where symptoms are present and levels are slightly outside the normal range) is seen in 4–10%.
Signs of hypothyroidism that people often attribute to anything but the thyroid:
- persistent fatigue and sleepiness, even after a full night's rest;
- feeling cold when others are comfortable;
- weight gain without changes in eating;
- dry skin, brittle hair and nails;
- slow thinking, poor memory ("brain fog");
- constipation;
- depression or emotional flatness.
A basic check requires only a TSH (thyroid-stimulating hormone) test. Normal TSH: 0.4–4.0 mIU/L, though lab reference ranges vary slightly. If TSH is above 4, discuss further testing with your doctor (free T4, anti-TPO antibodies).
When fatigue is depression
Depression is not just "feeling down". It's a medical condition, and fatigue is one of its defining features. A person with depression genuinely cannot get out of bed — not because of laziness, but because the biological resources simply aren't there.
Hallmarks of depression-related fatigue:
- fatigue doesn't improve with rest or sleep;
- alongside fatigue: emotional emptiness, loss of interest in things you used to enjoy;
- worst in the morning — depression frequently peaks in the early hours;
- sleep problems: difficulty falling asleep or early morning waking;
- reduced or absent appetite;
- negative thoughts, sense of hopelessness.
Important: depression responds well to treatment. If this description sounds familiar, it's a reason to see a psychiatrist or therapist — not to push through "just tiredness".
Which tests to get for chronic fatigue
A basic panel that helps a doctor find or rule out the main causes:
- Complete blood count (CBC) with differential — anaemia, infection, inflammation;
- Ferritin — iron stores (more informative than serum iron alone);
- TSH — thyroid function;
- Fasting glucose and/or HbA1c — rule out diabetes and prediabetes;
- 25(OH)D (vitamin D) — especially relevant in autumn and spring;
- Vitamin B12 — especially for vegetarians or those taking metformin;
- ESR, CRP — inflammatory markers;
- ALT, AST — liver function;
- Urinalysis — kidneys, infection.
If everything is normal but fatigue persists, a doctor may extend the workup: cortisol, sex hormones, autoimmune markers.
Not sure which doctor to see? Start by describing your symptoms to the assistant — it will help you get oriented. Or start with a GP — they'll run an initial workup and refer you to an endocrinologist, neurologist, or psychiatrist as needed.
Frequently asked questions
Why am I tired even after 8 hours of sleep?
Sleep duration is only one factor — quality matters just as much. Sleep apnoea (brief breathing interruptions), frequent arousals, or shallow sleep all cause fatigue despite a technically sufficient number of hours. Also check ferritin, TSH, and vitamin D: deficiencies impair recovery even with adequate sleep.
What is chronic fatigue syndrome?
Chronic Fatigue Syndrome (CFS, also called myalgic encephalomyelitis or ME/CFS) is a distinct medical condition in which fatigue persists for more than 6 months, doesn't resolve with rest, and is accompanied by a worsening of symptoms after any physical or mental exertion. The causes are not fully understood; diagnosis is made by excluding other conditions.
Can fatigue be a sign of cancer?
In some cancers, fatigue is an early symptom. However, it should not be the first thing you investigate. Start by ruling out common and easily treatable causes. If fatigue is accompanied by unexplained weight loss, swollen lymph nodes, or prolonged low-grade fever — see a doctor promptly.
Do vitamins actually help with fatigue?
They help — but only when there is a genuine deficiency. Taking vitamins "just in case" when levels are normal produces no meaningful benefit. Get tested first, then replace what's actually missing. With confirmed vitamin D or B12 deficiency, most people notice improvement within 4–8 weeks of supplementation.
Which doctor should I see for fatigue?
Start with a GP or general internist. They'll order baseline tests and, if needed, refer you to an endocrinologist (thyroid, diabetes), neurologist, psychiatrist, or haematologist. If you want to sort out the right specialist beforehand, use the doctor routing tool.
Is adrenal fatigue a real diagnosis?
"Adrenal fatigue" is a popular but medically unrecognised diagnosis. The actual adrenal condition — Addison's disease — is rare and diagnosed through specific tests. If you're offered treatment for "adrenal fatigue" without a standard workup, it's worth questioning the approach.
Symptomatica is an informational reference service. Not a medical service; does not diagnose or prescribe treatment. For any symptoms, please consult a doctor.