Labs to Test If You’re Chronically Fatigued

Feeling exhausted all the time is frustrating—especially when you’re eating well, sleeping, and still dragging through your days. Chronic fatigue can stem from many different root causes: blood sugar dysregulation, nutrient deficiencies, inflammation, hidden infections, or hormone imbalances. Instead of guessing, functional labs can give you answers.

Below, we’ll break down key labs to consider, along with the optimal ranges many functional medicine providers look for (not just “normal” ranges, which can be too broad).

1. Fasting Insulin

• Why test it? Even if your glucose looks normal, insulin resistance can make you feel sluggish, foggy, and prone to energy crashes.

• Functional range: Ideally <8 μIU/mL (some integrative practitioners prefer 2–5 μIU/mL).

• Average Cost: $7.00 - $16.00

2. Ferritin (Iron Storage)

• Why test it? Iron deficiency or overload both cause fatigue. Ferritin is the best measure of your body’s iron reserves.

• Functional range: 50–150 ng/mL (women often feel best between 70–100 ng/mL). Too low = deficiency; too high = possible inflammation or hemochromatosis.

• Average Cost: $7.00 - $21.00

3. Vitamin B12

• Why test it? Low B12 can cause fatigue, nerve issues, and brain fog.

• Functional range: >500 pg/mL (many practitioners prefer 700–1,000 pg/mL for optimal energy).

• Average Cost: $5.00 -$15.00

4. Methylation Markers

• Why test it? Methylation impacts detox, neurotransmitters, and energy. Genetic variants (like MTHFR) and nutrient imbalances can impair it.

• What to measure:

  • Homocysteine (see below under inflammation markers)

  • Folate, B12, B6 levels

  • Methylmalonic acid (MMA) as a sensitive marker of B12 activity

• Average Cost: $65.00

5. Cortisol (4-Point Salivary or Dried Urine Test)

• Why test it? Cortisol follows a daily rhythm—high in the morning, low at night. Dysregulation leads to “wired but tired” fatigue, sleep issues, and stress intolerance.

• Functional range:

  • Morning (30–60 min after waking): high/robust

  • Afternoon: gradual decline

  • Evening: low

  • Bedtime: lowest point of the day

• Average Cost: $169.00- $300.00

Flat or reversed curves often explain chronic fatigue symptoms.

6. Inflammation Markers

Chronic inflammation drains energy and drives fatigue. Key markers include:

• C-reactive protein (CRP): Functional range <1 mg/L

• Homocysteine: Functional range <9 μmol/L (optimal 5–7 μmol/L)

• Interleukin-6 (IL-6): Elevated IL-6 indicates systemic inflammation and immune dysregulation

• Average Cost: $25.00

7. Mitochondria & Nutrient Markers

Your mitochondria are the “power plants” of your cells. When they aren’t functioning well, fatigue sets in. Consider:

• Organic Acids Test (OAT): Shows mitochondrial activity, nutrient needs, and detox function

• Nutrient markers: Magnesium, zinc, selenium, CoQ10, L-carnitine, Vitamin D

• Functional ranges:

  • Vitamin D: 50–80 ng/mL

  • Magnesium (RBC): 5.0–6.5 mg/dL

  • CoQ10: upper half of lab range

• Average Cost: $200.00 - $280.00

8. Gut Health & Hidden Infections

Your gut and immune system are deeply connected to your energy. Chronic infections or dysbiosis can keep your body in “alert mode.”

• Stool testing: To assess microbiome balance, parasites, yeast, and digestive function

• Breath tests: For small intestinal bacterial overgrowth (SIBO)

• Bloodwork: Viral titers (EBV, CMV), Lyme/co-infections if clinical suspicion is high

• Average Cost: $330.00

Addressing these hidden infections can be a game-changer for chronic fatigue recovery.

Bringing It All Together

Chronic fatigue isn’t a one-size-fits-all problem—it’s a signal that something deeper is out of balance. Functional lab testing can help identify your unique root causes, whether it’s insulin resistance, nutrient depletion, inflammation, or stealth infections.

If you’re struggling with unexplained fatigue, working with a functional medicine provider can help you interpret these labs and design a targeted, personalized recovery plan.

References

1. Reaven GM. Insulin resistance and human disease: a short history. J Basic Clin Physiol Pharmacol. 1998.

2. Killip S, et al. Iron deficiency anemia. Am Fam Physician. 2007.

3. O'Leary F, Samman S. Vitamin B12 in health and disease. Nutrients. 2010.

4. Holtorf K. Diagnosis and treatment of hypothalamic-pituitary-adrenal (HPA) axis dysfunction. Integr Med (Encinitas). 2009.

5. Ridker PM. High-sensitivity C-reactive protein. Circulation. 2003.

6. Lord RS, Bralley JA. Clinical applications of urinary organic acids. Alternative Medicine Review. 2008.

7. Katz BZ, et al. Chronic fatigue syndrome after infectious mononucleosis in adolescents. Pediatrics. 2009.