Brain Fog Decision Tree.
The most common 'something's off' complaint, with no single workup. Five buckets — test each before stacking drugs.
Where brain fog actually comes from.
Almost every persistent brain-fog case sits in one of five buckets. Diagnose the bucket first; the intervention follows.
Sleep. The most common cause and the most overlooked. Less than 7 hours of quality sleep — measured by tracker, not memory — degrades cognition equivalent to 0.05 BAC. Most people with chronic brain fog have a sleep problem they've normalized.
Glucose dysregulation. Post-meal crashes, undiagnosed pre-diabetes, reactive hypoglycemia. A CGM for two weeks resolves the question.
Thyroid dysfunction. Subclinical hypothyroidism (TSH > 2.5 with low free T3) is a classic brain-fog cause that standard labs often miss.
Micronutrient deficits. B12 below 400 pg/mL, vitamin D below 30 ng/mL, low ferritin in menstruating women — each independently causes fog.
Environmental / situational. Mold exposure, undiagnosed ADHD, depression, COVID sequelae, alcohol, certain medications (statins, anticholinergics, antihistamines, SSRIs in some).
What to test, in order.
Cheap and high-yield first. Save expensive specialist workups for after the basics return negative.
Two weeks of objective sleep tracking. Oura, Whoop, or even Apple Watch. If you average <6.5 hours or your deep sleep is consistently <45 minutes, you have your answer.
Two weeks of CGM data. Stelo or Lingo. Look for post-meal peaks >160 mg/dL, dips below 65 mg/dL within 90 minutes of meals, or chaotic curves overall.
Full thyroid panel. Not just TSH. Free T3, free T4, reverse T3, TPO antibodies. Conversion failures hide from TSH-only screens.
Bloodwork basics + B12 + vitamin D + ferritin. See the Bloodwork Basics guide. Adds maybe $50 to a standard panel.
If all of the above are clean: consider ADHD evaluation (lifetime adult diagnosis is increasingly recognized), depression screening (PHQ-9), mold exposure history (VCS test or ERMI dust test), and a sleep study if snoring/witnessed apnea exists.
- Start with expensive specialist tests when you haven't tracked sleep or glucose.
- Try nootropics before the workup — they mask, they don't fix.
- Accept 'normal labs' as the final answer when the standard panel is thin.
Bucket-specific fixes.
Each bucket has a known intervention. Once you've identified the bucket, the protocol is short.
Related.
For educational purposes only. Persistent cognitive symptoms warrant evaluation by a qualified physician. Self-diagnosis and self-treatment can delay accurate diagnosis. This information does not substitute for personalized medical advice.