Overview.
Fatigue is the most common symptom reported to doctors, yet it's also the most commonly dismissed as 'just stress' without investigation. The reality: targeted blood work identifies a treatable cause in approximately 50% of chronic fatigue cases. Iron deficiency — even without full anaemia — is the single most common finding, followed by thyroid dysfunction, vitamin deficiencies, and blood sugar instability. A single fasting morning blood draw can test all of these and either pinpoint the problem or provide genuine reassurance.
Ranked biomarkers.
#1 Ferritin (Iron Stores)
Iron is required for haemoglobin (oxygen transport), myoglobin (muscle oxygen), and mitochondrial electron transport (ATP production). Low ferritin causes fatigue, brain fog, and exercise intolerance BEFORE haemoglobin drops — meaning you can be profoundly iron-depleted with a 'normal' CBC.
Optimal range: >50 ng/mL for energy. Standard lab 'normal' starts at 12-15 ng/mL — far below what the brain and muscles need for optimal function.
Key insight: Ferritin below 30 ng/mL causes significant fatigue in most people, yet labs report it as 'normal'. Women of reproductive age, vegetarians, and endurance athletes are most affected. Iron supplementation at ferritin <50 ng/mL improves fatigue even without anaemia.
#2 TSH (Thyroid-Stimulating Hormone)
Thyroid hormones set your metabolic rate — every cell in your body. Hypothyroidism is the second most common treatable cause of fatigue after iron deficiency. Affects 5-10% of adults (more common in women >40). Fatigue from hypothyroidism is a pervasive, 'bone-deep' exhaustion that doesn't improve with sleep.
Optimal range: 0.5-2.0 mIU/L (functional optimal). Standard 'normal' extends to 4.5 mIU/L, but many people feel significantly better with TSH in the lower half of the range.
Key insight: Subclinical hypothyroidism (TSH 4.0-10.0 with normal T4) is extremely common and frequently causes fatigue, even though many doctors dismiss it. If your TSH is >3.0 and you're exhausted, request Free T4 and TPO antibodies — early Hashimoto's may be present.
#3 Vitamin B12
B12 is essential for red blood cell production, myelin maintenance (nerve insulation), and DNA synthesis. Deficiency causes fatigue, cognitive slowing, numbness/tingling, and macrocytic anaemia. It's common in vegetarians/vegans, metformin users, older adults, and people with pernicious anaemia or malabsorption.
Optimal range: >400 pg/mL (functional optimal). Standard lab 'normal' starts at 200 pg/mL, but neurological symptoms can occur at levels of 200-400 pg/mL. Methylmalonic acid (MMA) is the definitive test for tissue-level deficiency.
Key insight: B12 deficiency develops slowly (liver stores last 3-5 years) and causes irreversible nerve damage if left untreated. If you've been vegetarian for years, on metformin, or over 60, check B12 proactively — don't wait for fatigue.
#4 Vitamin D (25-OH)
Vitamin D receptors exist in skeletal muscle, brain, and mitochondria. Deficiency causes muscle weakness, fatigue, and low mood — a triad frequently attributed to 'just getting older'. Affects ~40% of adults. Supplementation in deficient individuals reliably improves fatigue scores.
Optimal range: 40-60 ng/mL. Standard 'sufficient' starts at 30 ng/mL, but energy and mood benefits are seen when levels are pushed higher.
Key insight: Seasonal fatigue patterns (worse in winter, better in summer) are a strong clinical clue for vitamin D deficiency. If you feel dramatically more energetic during sunny holidays, test your vitamin D.
#5 Fasting Insulin / HbA1c
Blood sugar instability is an underrecognised cause of fatigue. Insulin resistance causes post-meal energy crashes ('food coma'), mid-afternoon slumps, and reliance on caffeine and sugar for energy. Even with 'normal' fasting glucose, elevated fasting insulin reveals the problem 5-10 years before diabetes diagnosis.
Optimal range: Fasting insulin <5 μIU/mL; HbA1c <5.3%. HOMA-IR <2.0. Standard 'normal' insulin (<25 μIU/mL) includes profoundly insulin-resistant individuals.
Key insight: If your energy crashes 2-3 hours after meals and you feel temporarily better after eating sugar — you likely have reactive hypoglycaemia driven by insulin resistance. This is the fatigue pattern that responds dramatically to dietary change (lower glycaemic index, more protein/fat, fewer refined carbs).
#6 Morning Cortisol
Cortisol follows a circadian rhythm: high in the morning (providing the energy to wake up) and low at night (allowing sleep). Flattened cortisol rhythm — from chronic stress, sleep disruption, or adrenal dysfunction — causes morning fatigue, afternoon crashes, and inability to wind down at night.
Optimal range: AM cortisol: 10-18 μg/dL (8 AM draw). <6 μg/dL warrants adrenal insufficiency evaluation. >23 μg/dL may indicate Cushing's syndrome.
Key insight: True adrenal insufficiency (Addison's disease) is rare (~1 in 10,000) but life-threatening if missed. The more common pattern is functional HPA axis dysregulation from chronic stress — where AM cortisol is low-normal and the daily rhythm is flattened. This is real and affects energy, but it's treated with lifestyle, not cortisol supplementation.
#7 Magnesium
Magnesium is required for ATP production — the fundamental energy currency of every cell. It's a cofactor for >300 enzymatic reactions. Deficiency causes fatigue, muscle cramps, insomnia, and anxiety. Affects ~20% of the population, more in stress, alcohol use, and diuretic therapy.
Optimal range: Serum: >2.0 mg/dL. RBC magnesium >5.0 mg/dL (more accurate for intracellular status).
Key insight: Serum magnesium is a poor test — it only drops when body stores are severely depleted (~70% gone). If you have unexplained fatigue with muscle cramps, insomnia, or anxiety and your serum magnesium is 'normal', request RBC magnesium or consider a therapeutic trial of magnesium glycinate (400mg/day).
#8 Haemoglobin / CBC
Haemoglobin carries oxygen from lungs to every tissue. Even mild anaemia (-1 g/dL below normal) measurably reduces exercise capacity, cognitive function, and energy. The CBC also reveals MCV (red cell size) — pointing toward iron deficiency (low MCV) or B12/folate deficiency (high MCV).
Optimal range: Men: >14.5 g/dL; Women: >13.0 g/dL (higher than standard lab minimums). MCV 80-100 fL (normal size).
Key insight: Don't stop at 'your CBC is normal'. If haemoglobin is technically normal but ferritin is low (<30 ng/mL), you have iron depletion without anaemia — and you're still fatigued because your tissues are iron-starved even though haemoglobin hasn't dropped yet.
How to test.
Request a fasting morning blood draw (7-9 AM): CBC with iron studies (ferritin, iron, TIBC), TSH, vitamin B12, vitamin D, fasting glucose + insulin, HbA1c, and magnesium (RBC if available). Add cortisol if fatigue is worst in the morning. This covers the vast majority of treatable causes in one draw.
FAQs.
My doctor says my blood work is normal but I'm exhausted — what now?
Ask specifically: what was my ferritin (not just haemoglobin), TSH, B12, and vitamin D? Many fatigue-relevant markers are either not tested (ferritin, B12, fasting insulin) or are 'normal' by overly wide lab ranges but suboptimal for function (TSH 3.5, ferritin 18, vitamin D 22 are all 'normal' but may explain your fatigue).
Which single test is most likely to explain my fatigue?
Ferritin. Iron deficiency is the single most common treatable cause of fatigue, especially in women of reproductive age. It causes fatigue at levels (15-30 ng/mL) that labs call 'normal'. If you could only test one thing, test ferritin.
Can stress alone cause this much fatigue?
Yes — chronic stress disrupts cortisol rhythm, impairs sleep quality, depletes magnesium, worsens insulin resistance, and suppresses thyroid conversion (T4→T3). But stress also makes you more susceptible to the deficiencies on this list. The answer isn't 'just reduce stress' — it's test first, treat what's found, then optimise lifestyle.
How quickly will I feel better after treating a deficiency?
Iron (if ferritin was low): 2-4 weeks for energy, 3-6 months for full repletion. Thyroid medication: 2-6 weeks. B12 injections: days to weeks. Vitamin D: 4-8 weeks. Blood sugar stabilisation (diet change): days. The fastest improvements come from iron and dietary changes; the slowest from vitamin D.
Verdict.
Fatigue is not a diagnosis — it's a symptom with measurable causes. These 8 biomarkers cover the most common and treatable drivers: iron depletion (ferritin), thyroid dysfunction (TSH), vitamin deficiencies (B12, D), metabolic instability (insulin, HbA1c), stress axis disruption (cortisol), cellular energy production (magnesium), and oxygen delivery (haemoglobin). A single morning blood draw tests all of them. If your fatigue has been dismissed as 'normal', it's time to look at the numbers — with optimal, not just standard, reference ranges.