Blood Tests for Fatigue: The 10-Test Workup That Finds the Cause
Persistent fatigue is the most common reason people visit their doctor — and one of the most frequently under-investigated. These 10 blood tests cover the most treatable causes of fatigue and should form the first-line workup.
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Get My Score →The Most Common Treatable Causes of Fatigue
Fatigue has dozens of causes — but the most common treatable ones are detectable with blood tests. The problem is that a "routine" blood test ordered for fatigue often includes only a CBC and CMP, missing the most commonly identified causes. A thorough fatigue workup requires 8–10 specific tests.
| Test | Condition It Rules Out | What to Look For |
|---|---|---|
| TSH | Hypothyroidism | TSH above 4.0 mIU/L — one of the most common reversible causes of fatigue |
| Ferritin | Iron deficiency without anaemia | Ferritin below 30 ng/mL — many people with fatigue have normal haemoglobin but depleted iron stores |
| 25-OH Vitamin D | Vitamin D deficiency | Below 30 ng/mL — associated with fatigue, muscle weakness, and mood disruption |
| Vitamin B12 | B12 deficiency | Below 300 pg/mL; confirm with MMA if borderline — neurological fatigue, brain fog |
| CBC (full blood count) | Anaemia, infection, leukaemia | Low haemoglobin; MCV helps identify type of anaemia |
| Fasting glucose and HbA1c | Diabetes/prediabetes | HbA1c above 5.7% — uncontrolled glucose impairs cellular energy production |
| Liver enzymes (ALT, AST) | Liver disease, NAFLD | Elevated ALT — subclinical liver disease frequently causes fatigue |
| eGFR and creatinine | Chronic kidney disease | eGFR below 60 — uraemia causes profound fatigue |
| CRP and ESR | Inflammatory or autoimmune condition | Markedly elevated CRP/ESR — suggests inflammatory cause requiring further workup |
| Cortisol (morning) | Adrenal insufficiency (rare but serious) | AM cortisol below 10 µg/dL with fatigue, hypotension, and low sodium suggests Addison's |
The Six-Test Fatigue Panel to Request
The Two Most Missed Causes: Ferritin and Vitamin D
Standard fatigue workups frequently miss iron deficiency without anaemia. A normal haemoglobin and CBC are mistakenly used to rule out iron as a cause — but ferritin can be depleted for months to years before anaemia develops. Studies show that raising ferritin from below 30 to above 50 ng/mL in women with fatigue and low-normal ferritin produces significant improvements in energy — even without anaemia. Vitamin D deficiency is the second most commonly missed cause: fatigue, muscle weakness, and depressed mood improve with correction in deficient individuals.
Ferritin and Vitamin D: The Two Most Overlooked Causes
When Blood Tests Are All Normal
If a thorough blood test panel is genuinely normal, the remaining causes of fatigue require different investigations: sleep studies (obstructive sleep apnoea is extremely common and rarely tested for), mental health evaluation (depression and anxiety cause significant fatigue), Epstein-Barr virus serology (post-viral fatigue), cortisol rhythm assessment (24-hour urinary free cortisol or saliva cortisol), and in some cases sleep diary or actigraphy. A normal blood panel is not a dead end — it redirects the search to causes that require different tests.
Medical Disclaimer
This article is for educational purposes only and does not constitute medical advice. Reference ranges, supplement dosages, and nutritional information mentioned are general educational guidance from published research—not personalised recommendations. Do not use this content to self-diagnose or self-treat any condition. Always consult a qualified healthcare provider before making any changes to your health regimen, medications, or supplements.
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