Chronic Fatigue: Which Blood Tests Should You Order?

"Chronic fatigue" as a symptom is common — most adults experience it at some point. Chronic Fatigue Syndrome (CFS), also called Myalgic Encephalomyelitis (ME/CFS), is a specific diagnosis with defined clinical criteria. Distinguishing the two matters because the treatment paths are different.

"Chronic fatigue" as a symptom: persistent fatigue lasting weeks to months. Often has identifiable medical contributors that can be diagnosed and treated through bloodwork (thyroid dysfunction, B12 deficiency, anemia, vitamin D deficiency, insulin resistance, sleep disorders, etc.). Most cases improve once the underlying contributor is identified and addressed.

ME/CFS (the clinical diagnosis): requires all four of these criteria per the 2015 Institute of Medicine criteria:

1. Substantial reduction in pre-illness activity level lasting 6+ months
2. Post-exertional malaise (worsening of symptoms after physical or mental exertion)
3. Unrefreshing sleep
4. Either cognitive impairment OR orthostatic intolerance

ME/CFS is a diagnosis of exclusion — it's diagnosed clinically after other causes of fatigue have been ruled out. That ruling-out process is what bloodwork does. A comprehensive workup catches the medical contributors that mimic CFS (and which are present concurrently in many CFS patients), so they can be treated independently.

Most people in our experience with unexplained chronic fatigue ARE NOT cases of ME/CFS — they have one or more treatable medical contributors that, once identified, account for most of their fatigue. Roughly 0.5-2.5% of US adults have ME/CFS, but many more have chronic fatigue from treatable causes. The blood workup determines which category you're in.

The standard chronic fatigue workup covers the most common medical contributors. This is what a thoughtful primary care physician, functional medicine practitioner, or chronic fatigue specialist would order:

Thyroid (TSH + Free T4 minimum; full panel preferred). Subclinical hypothyroidism and Hashimoto's thyroiditis are both common in chronic fatigue patients and dramatically underdiagnosed when TSH is the only test ordered. The Thyroid Complete Panel ($109.99) covers TSH + Free T4 + Free T3 + Reverse T3 + TPO Antibodies — catches conversion problems (low T3 with normal TSH and Free T4) which are common in chronic fatigue.

Iron status: Ferritin + Iron + TIBC. Iron deficiency without anemia is a leading cause of fatigue, especially in women. Ferritin below 30 ng/mL causes meaningful symptoms even though it's technically in the "normal" range; below 15 is unequivocally deficient. The Iron Panel covers all three markers.

Vitamin B12. Standard reference range starts at 200 pg/mL but functional deficiency commonly occurs at 200-400 pg/mL. Optimal for energy and cognitive function is 500-900. Especially worth checking in vegans/vegetarians, those over 60, and those on metformin or PPIs.

Vitamin D, 25-Hydroxy. ~40% of US adults are clinically deficient. Strongly associated with fatigue, mood symptoms, and muscle weakness. Optimal range is 40-60 ng/mL.

Hemoglobin A1c + Fasting Glucose. Pre-diabetes (A1c 5.7-6.4%) and insulin resistance commonly cause fatigue with afternoon energy crashes. The Diabetes Screening Panel ($59.99) bundles A1c, Fasting Glucose, and Fasting Insulin — fasting insulin rises before A1c becomes abnormal and catches the issue earlier.

Complete Blood Count (CBC). Screens for anemia, infection, and immune system abnormalities. Often included in any comprehensive workup.

Comprehensive Metabolic Panel (CMP). Liver function, kidney function, electrolytes. Catches contributors that might not be on your radar (chronic kidney disease causes profound fatigue; liver dysfunction does too).

hs-CRP (High-Sensitivity C-Reactive Protein). Inflammation marker. Often elevated in chronic fatigue patients; identifies whether systemic inflammation is contributing to symptoms.

ANA (Antinuclear Antibody) screen. Rules out (or identifies) autoimmune conditions like lupus, mixed connective tissue disease, and Sjögren's syndrome — all of which present with fatigue as a major symptom. Positive ANA warrants rheumatology referral for specific antibody testing.

Cortisol, AM. Drawn 7-9 AM. Persistently low morning cortisol (below 10 µg/dL) suggests HPA-axis dysfunction, which is associated with chronic fatigue and exercise intolerance. Persistently high may suggest active stress response or, rarely, Cushing's syndrome (requires further endocrine evaluation).

If the comprehensive workup above is unrevealing and fatigue persists, the following additional tests should be considered:

Lyme disease antibodies (in endemic areas). Lyme disease, especially chronic or post-treatment Lyme, commonly presents as chronic fatigue with cognitive symptoms, joint pain, and exercise intolerance. Worth testing if you live in or have visited Lyme-endemic areas (Northeast US, Mid-Atlantic, Upper Midwest, parts of California and the Pacific Northwest), have a history of tick exposure, or have other Lyme-suggestive symptoms.

EBV antibodies (VCA IgG, VCA IgM, EBNA-1). Epstein-Barr Virus (the cause of mononucleosis) is present in ~95% of adults as a latent infection. EBV reactivation patterns (high VCA IgG, low or absent VCA IgM, present EBNA-1) can contribute to chronic fatigue in some patients. Useful especially if fatigue onset followed a mono-like illness or persistent flu-like symptoms.

Celiac antibodies (tTG-IgA + total IgA). Celiac disease causes chronic fatigue through systemic inflammation and intestinal nutrient malabsorption (creating downstream B12, iron, D, magnesium deficiencies). About 1% of the US population has celiac and an estimated 60-80% are undiagnosed. Testing requires you to be eating gluten regularly.

Adrenal antibodies / Cosyntropin stimulation test. If morning cortisol is persistently low, addisonian disease (primary adrenal insufficiency) should be ruled out. This requires specialized testing through an endocrinologist, not a routine blood test.

Sex hormones. Low testosterone in men and perimenopausal hormonal shifts in women both cause significant fatigue. The Men's Hormone Panel ($159.99) or Women's Hormone Panel ($159.99) provides comprehensive evaluation.

Sleep evaluation (not bloodwork). Sleep apnea is one of the most common undiagnosed causes of chronic fatigue. If you snore, have witnessed apneas, wake up with morning headaches, or have daytime sleepiness despite adequate sleep duration, an at-home sleep study is worth doing in parallel with the bloodwork.

Heavy metals (lead, mercury). Rare but worth considering in cases with specific exposure history (older home with lead paint, frequent consumption of high-mercury fish, industrial exposure, specific hobbies).

If the workup reveals treatable contributors: address them per standard treatment protocols. Many people with chronic fatigue have 2-4 contributing issues that, when treated together, account for most of their fatigue. The recovery timeline varies — vitamin and mineral deficiencies typically improve within 3 months of correction; thyroid dysfunction within 6-12 weeks of medication; insulin resistance within 3-6 months of lifestyle change. Track symptoms over the treatment period to identify which interventions are helping.

If the workup is unrevealing or contributors are addressed but fatigue persists 6+ months: consider ME/CFS evaluation by a knowledgeable specialist. ME/CFS specialists are not common — academic medical centers (Stanford, Bateman Horne Center in Utah, Mount Sinai, Cornell Center for Excellence) have dedicated programs. The Solve ME/CFS Initiative (solvecfs.org) maintains a provider directory.

Key indicators that suggest ME/CFS specifically:

• Post-exertional malaise (PEM) — symptom worsening 12-48 hours after physical or mental exertion. This is the hallmark of ME/CFS and distinguishes it from other forms of chronic fatigue.
• Unrefreshing sleep — waking up just as tired as when you went to bed, regardless of sleep duration.
• Cognitive impairment — significant brain fog, word-finding difficulty, slow processing.
• Orthostatic intolerance — dizziness, racing heart, or pre-syncope on standing or being upright for extended periods (POTS-like presentation).

What to NOT do if PEM is present:

• Don't pursue graded exercise therapy. Older treatment recommendations included pushing through exercise, but modern data shows this worsens ME/CFS long-term. The current consensus is pacing — staying within your energy envelope and using "energy budgeting" to avoid PEM crashes.
• Don't accept "deconditioning" as the explanation. Some providers attribute all chronic fatigue to deconditioning and prescribe exercise. For people with true PEM, this is harmful. A knowledgeable provider will distinguish PEM from deconditioning.

Bringing your results to a provider: primary care doctor first for straightforward findings (treatable deficiencies, mild thyroid issues). Endocrinologist for complex thyroid or hormone issues. Rheumatologist for positive ANA. Sleep medicine for suspected sleep disorders. ME/CFS specialist if criteria are met. Functional medicine practitioner for integrative interpretation across multiple findings.