---
title: "Which blood tests help explain brain fog?"
type: symptom
url: "https://www.test-well.com/symptoms/brain-fog"
symptom: Brain fog
aliases:
  - trouble concentrating
  - memory problems
  - mental fog
  - can't focus
reviewed_at: 2026-09-06
review_status: reviewed
date_published: 2026-09-05
lab_fee_usd: 6
---

# Which blood tests help explain brain fog?

> 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.


## 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](https://www.test-well.com/conditions/brain-fog) — Thyroid, B12, vitamin D and metabolic markers when thinking feels cloudy.
- [Thyroid Problems](https://www.test-well.com/conditions/thyroid-problems) — TSH, Free T4/T3 and antibodies to evaluate an under- or over-active thyroid.
- [Anemia](https://www.test-well.com/conditions/anemia) — CBC, iron studies, ferritin, B12 and folate to find the cause of fatigue and low energy.

## Which tests, and what do they cost?

| Test or panel | Type | Price | All-in (one item + $6 fee) | Fasting | Results |
| --- | --- | --- | --- | --- | --- |
| [Chronic Fatigue Panel](https://www.test-well.com/tests/chronic-fatigue-panel) | Panel (9 tests) | $169.99 | $175.99 | Yes | 3–5 business days |
| [Complete Thyroid Panel](https://www.test-well.com/tests/thyroid-complete-panel) | Panel (4 tests) | $109.99 | $115.99 | No | 3–7 business days |
| [TSH (Thyroid Stimulating Hormone)](https://www.test-well.com/tests/tsh) | Test | $26.99 | $32.99 | No | 1–3 business days |
| [Vitamin B12](https://www.test-well.com/tests/vitamin-b12) | Test | $25.99 | $31.99 | No | 2–4 business days |
| [Ferritin](https://www.test-well.com/tests/ferritin) | Test | $24.99 | $30.99 | No | 1–3 business days |
| [Hemoglobin A1c](https://www.test-well.com/tests/hemoglobin-a1c) | Test | $21.99 | $27.99 | No | 2–4 business days |
| [Vitamin D, 25-Hydroxy](https://www.test-well.com/tests/vitamin-d) | Test | $41.99 | $47.99 | No | 1–3 business days |
| [CBC with Differential](https://www.test-well.com/tests/cbc-with-differential) | Test | $21.99 | $27.99 | No | 1–3 business days |
| [Cortisol (AM)](https://www.test-well.com/tests/cortisol-am) | Test | $26.99 | $32.99 | No | 2–4 business days |

Prices are cash-pay; every order adds one $6.00 lab processing fee. Physician authorization included.

## 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.

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---

Medically reviewed by Youmna DiStefano, MD (Medical Reviewer, TestWell); last reviewed 2026-09-06. Review policy: https://www.test-well.com/editorial-policy

Canonical page: https://www.test-well.com/symptoms/brain-fog
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Prices are cash-pay, in USD, and include physician authorization; every order adds one flat $6 lab processing fee. Educational content is not a diagnosis — discuss results with a clinician.