Symptom → lab tests
Which blood tests help explain brain fog?
Short answer
Brain fog — poor concentration, slow recall, mental fatigue — is commonly linked to thyroid dysfunction, vitamin B12 deficiency, low iron, blood-sugar swings and low vitamin D. The standard workup is TSH, vitamin B12, ferritin, hemoglobin A1c and vitamin D; a CBC and morning cortisol round it out when the first set is normal.
Medically reviewed by Youmna DiStefano, MD · Last reviewed September 2026
When should you see a clinician instead of ordering a test?
- Sudden confusion, trouble speaking, one-sided weakness or facial droop — call emergency services
- Confusion with a high fever or severe headache
- Progressive memory loss that affects daily function — see a clinician for a full evaluation
Why these tests for brain fog?
Several very common, correctable problems affect thinking speed and focus. Low thyroid hormone slows cognition; vitamin B12 deficiency can cause memory and concentration problems (and neurological symptoms) before it causes anemia; low ferritin limits oxygen delivery; and blood-sugar swings — caught by A1c and fasting insulin — produce the classic mid-afternoon fog.
None of these tests diagnoses brain fog, but together they find or exclude the causes a clinician would check first. Sleep, medications, stress and depression are the non-lab causes to weigh alongside the results.
Which conditions do these tests help evaluate?
- Brain Fog — Thyroid, B12, vitamin D and metabolic markers when thinking feels cloudy.
- Thyroid Problems — TSH, Free T4/T3 and antibodies to evaluate an under- or over-active thyroid.
- Anemia — CBC, iron studies, ferritin, B12 and folate to find the cause of fatigue and low energy.
Which tests, and what do they cost?
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Chronic Fatigue PanelSaves $77.92 vs ordering the tests separately | Panel · 9 tests | $169.99 | Yes | 3–5 business days |
| Complete Thyroid PanelSaves $10.97 vs ordering the tests separately | Panel · 4 tests | $109.99 | No | 3–7 business days |
| TSH (Thyroid Stimulating Hormone) | Test | $26.99 | No | 1–3 business days |
| Vitamin B12 | Test | $25.99 | No | 2–4 business days |
| Ferritin | Test | $24.99 | No | 1–3 business days |
| Hemoglobin A1c | Test | $21.99 | No | 2–4 business days |
| Vitamin D, 25-Hydroxy | Test | $41.99 | No | 1–3 business days |
| CBC with Differential | Test · 14 biomarkers | $21.99 | No | 1–3 business days |
| Cortisol (AM) | Test | $26.99 | No | 2–4 business days |
Common questions
Can a normal TSH still leave a thyroid cause for brain fog?
TSH is the screen; if it's normal but symptoms are strongly thyroid-like, Free T4 and Free T3 add detail. Most people with a normal TSH do not have a thyroid cause.
Is a B12 level of 250 pg/mL enough?
It's in the low-normal zone many labs report as 200–300. Values there can be functionally low; a clinician may add methylmalonic acid to clarify. Discuss borderline results rather than self-treating.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.