Brain Fog Test: The Blood Panel
Most GPs will only run TSH and CBC by default — you need to specifically request the rest. Standard lab ranges show if you are in a disease state; cognitive optimal ranges show if your brain has what it needs to function at peak capacity.
The Tiered Investigation
Tier 1 — Request First
Cheapest, highest yield. Any GP can order these. Covers the most common reversible causes.
Tier 2 — If Tier 1 Normal
Fog persists after Tier 1 is unrevealing. Some require a specialist referral.
Tier 3 — Specialist
Requires clinical context. Share your symptom pattern with the specialist.
"Normal" vs Optimal
Standard lab ranges are based on avoiding disease, not optimizing function. A TSH of 3.5 is "normal" but many people feel foggy until it's below 2.0.
TSH
Normal: 0.4-4.5 → Optimal: 0.5-2.0
Ferritin
Normal: >12 → Optimal: 50-150
B12
Normal: >200 → Optimal: >500
How to Use This With Your Doctor
Print this section or copy to your phone. Say: "I've been experiencing persistent brain fog and found an evidence-based clinical resource suggesting these tests. Could we run them?" If your GP refuses the full thyroid panel, ask: "Could we run it once to rule it out? TSH alone can miss subclinical dysfunction."
The Core Biomarkers Database
| Biomarker | Optimal Range | Clinical Significance | Action |
|---|---|---|---|
| TSH (Thyroid Stimulating Hormone) | 1.0–2.0 mIU/L (optimal) | Primary thyroid screening marker. Values >2.5 may cause fog even if 'normal' | Detail → |
| Free T3 | 3.0–4.0 pg/mL | Active thyroid hormone that directly affects brain function | Detail → |
| Free T4 | 1.0–1.5 ng/dL | Thyroid hormone precursor — low levels indicate hypothyroidism | Detail → |
| TPO Antibodies | <35 IU/mL | Elevated in Hashimoto's thyroiditis — autoimmune thyroid attack | Detail → |
| Thyroglobulin Antibodies | <20 IU/mL | Second marker for autoimmune thyroid disease | Detail → |
| Reverse T3 | 10–24 ng/dL | Inactive T3 form — elevated during illness, stress, or low-calorie states | Detail → |
| C-Reactive Protein | <3.0 mg/L | General inflammation marker — less sensitive than hs-CRP | Detail → |
| IL-6 (Interleukin-6) | <1.8 pg/mL | Pro-inflammatory cytokine elevated in Long COVID and chronic inflammation | Detail → |
| TNF-α (Tumor Necrosis Factor Alpha) | <8.1 pg/mL | Key inflammatory cytokine — elevated crosses blood-brain barrier | Detail → |
| ESR (Erythrocyte Sedimentation Rate) | <20 mm/hr | Non-specific inflammation marker — elevated in autoimmune conditions | Detail → |
| D-dimer | <0.5 μg/mL | Clotting marker — elevated in Long COVID microclots | Detail → |
| ANA (Antinuclear Antibodies) | Negative (<1:40) | Screening for autoimmune disease — positive in lupus, Sjögren's, RA | Detail → |
| Anti-dsDNA Antibodies | Negative | Specific for systemic lupus erythematosus (SLE) | Detail → |
| tTG-IgA (Tissue Transglutaminase) | Negative (<4 U/mL) | Celiac disease screening — gluten triggers neuroinflammation | Detail → |
| Rheumatoid Factor | <14 IU/mL | Elevated in rheumatoid arthritis and other autoimmune conditions | Detail → |
| Serum Tryptase | <11.5 ng/mL | Mast cell activation marker — elevated in MCAS | Detail → |
| DAO (Diamine Oxidase) Levels | >10 U/mL | Low DAO = impaired histamine breakdown = histamine intolerance | Detail → |
| Plasma Histamine | <1.0 ng/mL | Direct histamine measurement — elevated causes brain fog, flushing | Detail → |
| Prostaglandin D2 | <890 pg/mL | Mast cell mediator — elevated in MCAS and allergic reactions | Detail → |
| N-Methylhistamine (24hr Urine) | <200 μg/g creatinine | Histamine metabolite — more stable than plasma histamine | Detail → |
Showing first 20 biomarkers. View complete diagnostic guide →
PART XI
Medical Rule-Outs
Diagnostic pathways for 64 causes
EXPLORE
All 64 Causes
Each with specific test recommendations
How to use this panel
Request these specific markers from your primary care physician or a private lab. Note that "Optimal" targets may differ from standard lab reference ranges. Always interpret results in conjunction with clinical symptoms and under professional supervision.
Learn about our reference range methodology →This information is for educational purposes only. Always consult with a qualified healthcare professional.