Brain Fog Glossary
491 terms explained in plain language. Use this index to understand the clinical terminology used across causes, tests, and treatment explainers.
25-OH Vitamin D
Severe deficiency doubles dementia risk
A1c + fasting glucose context review
This route is for the situation where HbA1c and fasting glucose do not fully explain a strong post-meal or fasting crash pattern.
AASP Sensory Profile
Adult sensory processing patterns
ACB
Now calculate the Anticholinergic Burden.
acetylcholine
The primary neurotransmitter for memory, learning, and attention.
acetylcholinesterase
The enzyme that breaks down acetylcholine after nerve signal transmission. Organophosphate and carbamate pesticides inhibit this enzyme, causing acetylcholine to accumulate and overstimulate nerves. The mechanism behind pesticide-induced cognitive symptoms.
actigraphy
A wearable sleep-wake tracker used over days to weeks to show actual timing patterns when sleep logs and memory are unreliable.
Active B12 (Holotranscobalamin)
More accurate than serum B12 for cellular deficiency
Adaptogen
A plant-derived compound marketed as helping the body adapt to stress. Some adaptogens have small stress-response trials, but the category is not a diagnosis or a guarantee of endocrine benefit.
adenosine
A chemical that builds up during waking hours, creating sleep pressure.
ADHD
ADHD is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
Adrenal insufficiency
A medical disorder where the body does not produce enough cortisol. It can cause fatigue, dizziness, low blood pressure, and can become an emergency if severe.
AHI
Apnea-Hypopnea Index, a sleep-study metric used when sleep apnea is part of the ADHD differential.
Air
Air can contribute to brain fog.
Alcohol
Alcohol can contribute to brain fog.
allopregnanolone
A neurosteroid derived from progesterone that modulates GABA receptors, affecting mood, anxiety, and cognition. Abnormal sensitivity to allopregnanolone fluctuations during the menstrual cycle may underlie PMDD symptoms.
alpha rhythm
Brain waves in the 8-13 Hz frequency range, associated with relaxed alertness, attention readiness, and cortical arousal regulation. Reduced posterior alpha is consistently found in brain fog patients across Long COVID, ME/CFS, and fibromyalgia studies (Babiloni 2024). Think of alpha as your brain's 'ready' signal — when it's low, you're not cognitively primed.
AM Cortisol (8am)
Morning cortisol explainer focused on the timed 8am draw clinicians usually use for baseline adrenal-context questions.
AMH
Anti-Müllerian hormone — a hormone produced by ovarian follicles that reflects ovarian reserve. Elevated AMH is common in PCOS and correlates with follicle count. Used alongside other criteria to support PCOS diagnosis.
Amygdala
A threat-detection and emotion-processing region of the brain that becomes more reactive under stress.
amyloid
A protein that can accumulate in the brain when the glymphatic system isn't clearing waste effectively. Associated with Alzheimer's disease. Deep sleep is essential for amyloid clearance.
Amyloid-beta
A protein fragment involved in normal brain biology that can accumulate abnormally when clearance is impaired; it is often discussed in the context of Alzheimer disease and sleep-linked waste clearance.
ANA
Antinuclear Antibodies.
ANA (Antinuclear Antibodies)
Screening for autoimmune disease — positive in lupus, Sjögren's, RA
anaerobic threshold
The activity intensity above which the body shifts into a less sustainable energy state. In ME/CFS and Long COVID, crossing it can contribute to post-exertional worsening.
Androgen
A hormone in the testosterone family. In adults, androgens influence libido, muscle maintenance, recovery, body hair, and parts of motivation and cognitive drive.
Anemia
Anemia can contribute to brain fog.
Anti-dsDNA Antibodies
Specific for systemic lupus erythematosus (SLE)
anticholinergic
Medications that block acetylcholine — the neurotransmitter for memory and learning.
Anticholinergic Burden Score
Score >3 associated with cognitive decline
Anxiety
Anxiety can contribute to brain fog.
apnea
Sleep apnea — repeated pauses in breathing during sleep that drop oxygen levels and fragment sleep architecture.
AQ-10 Screening
Autism spectrum screening questionnaire
Aromatase
An enzyme that converts testosterone into estradiol. Aromatase activity tends to rise with higher body-fat levels, which can lower bioavailable testosterone.
ASL-MRI
Arterial Spin Labeling.
ASL-MRI (Arterial Spin Labeling)
Non-invasive brain blood flow measurement
ASRS-v1.1
The Adult ADHD Self-Report Scale, a short screening tool used to identify people who may need full ADHD evaluation.
ASRS-v1.1 (ADHD)
Patient-facing ADHD screener route matching the ASRS-v1.1 wording used in results cards and clinician conversations.
ASRS-v1.1 Screener
Adult ADHD Self-Report Scale — 6-question screener
Atomoxetine
A non-stimulant ADHD medication that works primarily through norepinephrine reuptake inhibition.
Attention fragmentation
A pattern of repeatedly broken focus caused by interruptions, notification anticipation, or rapid switching between digital inputs.
Attention Restoration Theory
A theory suggesting that natural environments restore directed attention because they engage the mind gently rather than demanding constant cognitive control.
Autism
Autism can contribute to brain fog.
Autoimmune
Autoimmune can contribute to brain fog.
autophagy
The cell's self-cleaning process — damaged components are broken down and recycled. Triggered by fasting, exercise, and sleep. Impaired autophagy leads to cellular debris accumulation and cognitive decline.
B12
Holotranscobalamin.
B2
Magnesium and riboflavin.
Bartonella
Bartonella can contribute to brain fog.
Bartonella IgG/IgM
Bartonella infection screening
Bartonella PCR
Direct detection of Bartonella DNA
Baseline Cognitive Assessment
A baseline screen helps document that the problem is measurable, track change over time, and decide when formal neuropsychology is worth the extra effort.
BBB
Blood-brain barrier — the selective membrane protecting the brain from harmful substances in the bloodstream. Damage to the BBB is a key driver of brain fog.
BDNF
Brain-derived neurotrophic factor — a protein that promotes neuron growth, survival, and new connections.
Beighton Score
Joint hypermobility assessment for EDS/HSD
benzodiazepine
A class of sedative medications (Valium, Xanax, Ativan) that enhance GABA activity. Effective short-term for anxiety but cause cognitive impairment, memory problems, and dependence with regular use.
Bergen Social Media Addiction Scale
A short questionnaire used to screen for problematic social-media use by asking about salience, tolerance, withdrawal, conflict, relapse, and mood modification.
Bile acid sequestrant
A drug class that binds bile acids in the gut. Examples include cholestyramine and colesevelam; some mold-focused clinicians use them as binders.
bioaccumulation
Process by which mercury concentration increases up the food chain.
biomarker
A measurable substance in the body that indicates a biological state or condition. Blood biomarkers like ferritin, hs-CRP, and TSH help identify the root cause of brain fog objectively.
biopsychosocial model
A framework viewing health as the product of biological, psychological, and social factors interacting — not just physical disease.
Blood Mercury
Mercury exposure — half-life ~70-80 days.
blood mercury level
Measurement of mercury in whole blood.
blood pooling
In POTS and dysautonomia, blood accumulates in the lower body when standing due to poor venous return. Causes reduced cerebral perfusion and brain fog. Visible as purple/red discoloration in feet (acrocyanosis). Treated with compression garments and counter-maneuvers.
Blood Sugar Assessment
This bundle is more useful than a single glucose marker when the story suggests post-meal crashes, normal average labs with variability, or early insulin resistance.
blood-brain barrier
A selective membrane that controls what enters the brain from the bloodstream.
Blue light
Short-wavelength visible light from screens and LEDs that can shift circadian timing when exposure is concentrated late in the evening.
Box breathing
A structured breathing pattern with equal-length inhale, hold, exhale, and hold phases, often used to reduce physiological arousal.
Brain MRI
Structural neuroimaging used to evaluate red flags and differential neurological causes.
Brain rot
An emerging informal term for the dulled, low-depth mental state linked to compulsive low-quality digital content consumption. It is not a formal medical diagnosis.
Breath test
The primary diagnostic test for SIBO. You drink a sugar solution (lactulose or glucose) and breathe into collection tubes over 2-3 hours. Elevated hydrogen, methane, or hydrogen sulfide indicates bacterial overgrowth.
BRIEF-A
Behavior Rating Inventory of Executive Function
Burnout
Burnout can contribute to brain fog.
butyrate
A short-chain fatty acid produced when gut bacteria ferment dietary fibre. Strengthens the gut lining, reduces inflammation, and crosses the blood-brain barrier to directly support neurons.
C-Reactive Protein
General inflammation marker — less sensitive than hs-CRP
C4a Complement
Inflammatory marker elevated in CIRS and Lyme
Caffeine
Caffeine can contribute to brain fog.
Carbon monoxide
A colorless, odorless gas that can impair thinking, trigger headaches, and become dangerous or life-threatening with ongoing exposure.
CBC
Complete blood count — a basic blood panel that measures red cells, white cells, and platelets.
CBC + CMP
Baseline panel combining complete blood count and metabolic chemistry for broad screening context.
CBC with Differential
Core blood count panel used to review white cell patterns, hemoglobin, and platelet context.
CBT
Cognitive behavioral therapy, a structured psychological treatment that can help when problematic internet or social media use becomes compulsive.
CBT for ADHD
A structured, skills-based cognitive behavioral approach adapted for executive dysfunction rather than generic talk therapy.
CBT-I
Cognitive Behavioral Therapy for Insomnia — the first-line treatment for chronic insomnia, focused on habits, timing, and sleep-related arousal rather than sedating the brain.
CCI
Craniocervical instability.
CDT
Carbohydrate-Deficient Transferrin.
CDT (Carbohydrate-Deficient Transferrin)
Specific marker for chronic alcohol consumption
Celiac
Celiac can contribute to brain fog.
Central Sensitisation Inventory
Screens for central sensitization syndromes
cerebral hypoperfusion
Reduced blood flow to the brain. In POTS, this occurs on standing when blood pools in the legs instead of reaching the brain. Directly causes brain fog, difficulty concentrating, and cognitive slowing. Measured via transcranial Doppler ultrasound.
Cervical
Cervical can contribute to brain fog.
CGM
Continuous glucose monitor. A wearable sensor that shows glucose trends in near real time and can reveal variability that HbA1c misses.
CGM (Continuous Glucose Monitor)
14-day glucose variability tracking — reveals hidden spikes
chelation therapy
Medical treatment using drugs (DMSA, DMPS, or EDTA) that bind heavy metals in the blood for excretion.
Chemobrain
Chemobrain can contribute to brain fog.
cholestyramine
A bile acid sequestrant originally used for cholesterol. In CIRS/mold illness protocols, used to bind biotoxins in the gut and prevent reabsorption (enterohepatic recirculation). Requires careful timing away from other medications.
chronicity
The tendency of a condition to persist over time. In post-viral illness, chronicity means symptoms can last for months rather than days or weeks and usually require active management rather than passive waiting.
circadian
Relating to the body's ~24-hour internal clock. Circadian disruption (irregular sleep times, night-shift work, blue light at night) impairs melatonin production, cortisol rhythm, and cognitive performance.
Circadian anchoring
Using a consistent wake time and morning light exposure to stabilize the body clock so melatonin timing and alertness become more predictable.
circadian drift
A repeating pattern where sleep time keeps sliding later or becomes irregular enough that the brain never gets a stable wake cue.
Circadian rhythm
The body’s internal 24-hour timing system that affects alertness, sleep, hormone timing, and next-day cognitive function.
CIRS
Chronic Inflammatory Response Syndrome — a proposed condition where biotoxins (from mold, Lyme, etc.) trigger persistent innate immune activation. Characterized by multiple symptoms including brain fog, fatigue, and sensitivity to environments. Controversial; not recognized in mainstream guidelines.
CKD
Chronic kidney disease, a common metabolic-vascular contributor to cognitive symptoms.
Co-infection Panel
Tests for Babesia, Bartonella, Anaplasma, Ehrlichia
CO₂ Monitor
High CO₂ = poor ventilation = cognitive impairment
Cognitive reserve
The brain’s resilience against injury, aging, or disease. Higher reserve means a person may function better for longer despite the same biological stressor.
Colesevelam
A bile-acid sequestrant prescription medication used mainly for cholesterol and diabetes care. Some CIRS clinicians use it as a gentler alternative to cholestyramine.
Collateral history
Information from a parent, partner, sibling, report card, or older records that helps establish the childhood pattern.
Cortisol
Cortisol is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
cortisol awakening response
The normal rise in cortisol during the first 30 to 45 minutes after waking. A blunted pattern can show up in chronic stress and poor recovery states.
counter-maneuvers
Physical movements to increase blood return to the heart in POTS and orthostatic intolerance. Examples: leg crossing with thigh tensing, squatting, toe raises, fist clenching. Used before and during standing to prevent symptoms.
CPAP
Continuous Positive Airway Pressure — the gold-standard treatment for obstructive sleep apnea.
CPET
Cardiopulmonary exercise test — measures oxygen consumption, carbon dioxide output, heart rate, and ventilation during exercise. A two-day CPET protocol can objectively demonstrate post-exertional malaise in ME/CFS by showing reduced capacity on day 2. Gold standard for documenting exercise intolerance.
Cushing's syndrome
A medical disorder caused by pathologically high cortisol exposure. It requires clinician-guided diagnosis and should not be confused with everyday stress.
cytokine
Chemical messengers released by immune cells. Pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) drive brain fog by disrupting neuron communication. Anti-inflammatory cytokines help resolve it.
D-dimer
Clotting marker — elevated in Long COVID microclots
D-lactic acidosis
A metabolic condition where D-lactate-producing bacteria in the gut cause neurological symptoms including brain fog, confusion, and slurred speech. Linked to SIBO and probiotic overuse.
DAO
Diamine oxidase — the enzyme that breaks down histamine in the gut.
DAO (Diamine Oxidase) Levels
Low DAO = impaired histamine breakdown = histamine intolerance
deconditioning
A state of reduced physical fitness from prolonged inactivity.
dendritic
Relating to dendrites — the branch-like extensions of neurons that receive signals from other neurons. Dendritic sprouting (growth of new branches) is a sign of healthy neuroplasticity.
DePaul Symptom Questionnaire
A validated symptom questionnaire used in ME/CFS research and clinical workups to capture PEM, sleep, pain, autonomic, and cognitive patterns in a structured way.
Depression
Depression is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
Dexamethasone suppression test
A cortisol test where a small dose of dexamethasone is taken the night before blood work to check whether cortisol production suppresses normally.
DHEA-S
A long-acting adrenal hormone often used alongside cortisol to add context to chronic stress and HPA-axis discussions.
Diabetes
Diabetes can contribute to brain fog.
DIAGNOSTIC
THE COLLAR TEST.
Digital
Digital is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
Digital Motion X-ray
Real-time imaging of cervical instability
DII
Dietary Inflammatory Index — a literature-derived scoring system that rates the inflammatory potential of a diet based on 30+ nutrients. Higher DII = more pro-inflammatory. UK Biobank research proves DII predicts anxiety, depression, sleep disorders, and dementia risk through measurable blood markers.
DIVA-5
A structured diagnostic interview used in some adult ADHD assessments.
Doomscrolling
Compulsive, repetitive consumption of distressing or low-value content that keeps attention captured without restoring clarity.
Dopamine
A neurotransmitter involved in reward, motivation, and attention regulation. ADHD treatment often targets dopamine and norepinephrine pathways.
drift diffusion model
A computational model describing how the brain accumulates information when making decisions. In Long COVID, reduced drift rate and altered decision boundaries show slowed information processing and need for more evidence before deciding.
DRSP
Daily Record of Severity of Problems — a validated daily symptom tracking tool for diagnosing PMDD. Requires tracking for at least two menstrual cycles to confirm the pattern of luteal-phase symptoms with follicular-phase relief.
DSM-5
The fifth edition of the Diagnostic and Statistical Manual of Mental Disorders, which lays out current ADHD diagnostic criteria.
DTI MRI (Diffusion Tensor Imaging)
Advanced brain imaging — detects concussion damage
Dual N-Back
A working-memory training task where you track visual and auditory stimuli across multiple steps back in time. It is one of the few brain-training tasks with some transfer-effect evidence.
DUTCH test
A specialty dried urine hormone test that estimates cortisol patterns and sex-hormone metabolites over a day. It is not a standard first-line medical test.
dysautonomia
Dysfunction of the autonomic nervous system — the automatic controller of heart rate, blood pressure, digestion, and temperature.
dysbiosis
An imbalance in gut bacteria — too many inflammatory species, too few protective ones. Drives systemic inflammation, impairs nutrient absorption, and disrupts neurotransmitter production.
EA
AND Early Antigen.
EBV
EBV can contribute to brain fog.
EBV Reactivation Panel
This panel is not a routine answer for every Long COVID case, but it can help when the history suggests latent-virus reactivation may be adding to the fog pattern.
EDS
Ehlers-Danlos Syndrome.
eGFR
Estimated glomerular filtration rate, a standard measure of kidney function.
Electrolytes
Electrolytes can contribute to brain fog.
elimination diet
A structured approach to identifying food triggers: remove suspected foods for 2–4 weeks, then reintroduce one at a time while monitoring symptoms. The gold standard for finding diet-driven brain fog.
ELISA (Lyme)
Lyme screening — high false negative rate
EMF
Electromagnetic fields generated by electrical devices and wireless systems. Brain-fog relevance remains controversial and low-priority compared with better-supported triggers.
EncephalApp
A Stroop-style app used to help screen for covert hepatic encephalopathy.
Endometriosis
Endometriosis can contribute to brain fog.
energy envelope
A pacing concept in ME/CFS where you plan daily activity to stay within the amount of energy your body can reliably tolerate without triggering a crash.
EPDS
Take the Edinburgh Postnatal Depression Scale.
EPO
Damaged kidneys produce less erythropoietin.
Epworth
Epworth Sleepiness Scale — an 8-question self-assessment measuring daytime sleepiness. Score your likelihood of dozing in different situations (reading, watching TV, as a passenger). Score ≥10 = excessive daytime sleepiness, suggesting a sleep disorder.
Epworth Sleepiness Scale
A short questionnaire that estimates how likely you are to fall asleep in ordinary situations; high scores suggest daytime sleepiness is real, not imagined.
ERMI
Environmental Relative Moldiness Index.
ERMI (Environmental Relative Moldiness Index)
Home mold DNA test — identifies problematic species
ESR
Erythrocyte Sedimentation Rate.
ESR (Erythrocyte Sedimentation Rate)
Non-specific inflammation marker — elevated in autoimmune conditions
Estradiol
Primary estrogen — decline impairs hippocampal function
Executive dysfunction
Difficulty organizing, initiating, sequencing, and following through, even when the task is understood.
Expressive writing
A structured writing exercise where you write openly about a stressful experience for a short, fixed block of time.
Fasting Glucose
Higher fasting glucose impairs executive function
Fasting Insulin
Insulin resistance marker often paired with fasting glucose and HbA1c.
Fatigue
Fatigue is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
feature importance
In machine learning, a score indicating how much a variable contributes to predicting an outcome. Higher feature importance means the model relied more heavily on that variable when separating groups or making predictions.
Fecal Calprotectin
Gut inflammation marker — elevated in IBD, infections
Ferritin
A blood marker reflecting iron stores. Low ferritin can worsen fatigue, restless legs, and in some cases attention symptoms.
FIB-4
A non-invasive liver-fibrosis estimate derived from age, AST, ALT, and platelet count.
Fibromyalgia
Fibromyalgia is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
Finger Tapping Test
Simple motor-speed screening task sometimes used in cognitive and neurological assessments.
Flexion-Extension Imaging
Dynamic X-ray for cervical spine movement
FODMAP
Fermentable Oligosaccharides, Disaccharides, Monosaccharides, And Polyols - short-chain carbohydrates that feed bacteria. Temporarily reducing FODMAPs can relieve SIBO symptoms while treating the underlying overgrowth.
folate
Vitamin B9 — essential for methylation, DNA repair, and neurotransmitter production.
Folate (Serum or RBC)
Essential for methylation and neurotransmitter synthesis
follicular phase
The first half of the menstrual cycle, from menstruation to ovulation (roughly days 1-14). Estrogen rises during this phase. In PMDD, symptoms are absent or minimal during the follicular phase — this contrast is diagnostic.
FoMO
Fear of missing out. In digital-overload contexts it can make silence, batching, and distance from the phone feel harder than they should.
Food sensitivity
Food sensitivity can contribute to brain fog.
Free T3
The active form of thyroid hormone that directly affects brain metabolism.
Free T4
The storage form of thyroid hormone.
Free Testosterone
Bioavailable testosterone — more clinically relevant than total
FSH
Follicle-Stimulating Hormone.
FSH (Follicle-Stimulating Hormone)
Elevated >25 mIU/mL suggests perimenopause/menopause
FSH / Estradiol
This pair is mainly useful when the story itself contains cycle disruption, menopausal transition, fertility treatment, postpartum change, or clinician concern about estrogen status.
GABA
Gamma-aminobutyric acid — the brain's main calming neurotransmitter. Balances excitatory signals. Benzodiazepines and alcohol artificially boost GABA, which is why withdrawal causes rebound anxiety and fog.
GAD-7
Patient-facing anxiety screener route matching the short GAD-7 label used in results and referrals.
GAD-7 (Anxiety)
Validated anxiety symptom screener used to support differential assessment in clinical context.
GGT
Gamma-Glutamyl Transferase.
GGT (Gamma-Glutamyl Transferase)
Liver enzyme — elevated with alcohol, medications
GI-MAP
Stool DNA Test.
GI-MAP (Stool DNA Test)
Detects pathogens, parasites, dysbiosis, digestive markers
GLP-1
Glucagon-like peptide-1, a hormone pathway targeted by drugs such as semaglutide and tirzepatide.
Glucose Breath Test
Alternative SIBO test — only detects proximal overgrowth
glutamate
The brain's primary excitatory neurotransmitter. Essential for learning, but excess glutamate is neurotoxic. MSG sensitivity and neuroinflammation can cause glutamate overload.
Glutathione (RBC)
Master antioxidant — low indicates oxidative stress
Glycation
A chemical process where excess glucose sticks to proteins and tissues, contributing to long-term metabolic and vascular damage.
glymphatic
The brain's waste-clearance system, most active during deep sleep.
glyphosate
The active ingredient in Roundup and other herbicides — the world's most widely used pesticide. Disrupts the gut microbiome by affecting the shikimate pathway in bacteria. Debate continues about neurological effects at typical exposure levels.
Gut
Gut is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
Hair Cortisol
3-month cortisol average — best for chronic stress assessment
hair mercury
Mercury concentration in hair, reflecting average exposure over the growth period of the hair sample (approximately 1 cm per month).
Hashimoto
Hashimoto's thyroiditis — an autoimmune condition where the immune system attacks the thyroid gland.
HbA1c
Glycated haemoglobin — measures average blood sugar over 2–3 months.
HbA1c + Fasting Insulin
This panel is more useful than HbA1c alone when the story suggests glucose variability, reactive symptoms, or insulin-resistance overlap.
HBOT
Hyperbaric oxygen therapy — breathing pure oxygen in a pressurised chamber. Promotes neuroplasticity and reduces neuroinflammation. The strongest evidence is for post-COVID brain fog (60-session protocols at 2.0 ATA).
Heart Rate Monitoring
Track HR response to position changes
Heavy Metal Panel
Blood/urine screening for lead, mercury, arsenic, cadmium
Hepatic encephalopathy
Brain dysfunction caused by liver failure or portal-systemic shunting that allows neurotoxic substances to affect cognition.
Histamine
Histamine is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
HLA-DR Genotype
24% have mold-susceptible HLA types
HOMA-IR
Homeostatic Model Assessment of Insulin Resistance — calculated from fasting glucose and insulin levels. Higher values indicate greater insulin resistance. Often elevated in PCOS, metabolic syndrome, and prediabetes. Improving HOMA-IR through lifestyle changes can reduce brain fog.
Homocysteine
Methylation marker linked to B-vitamin status and vascular risk context.
HPA axis
Hypothalamic-pituitary-adrenal axis — the brain-to-adrenal gland communication system that controls your stress response.
HRV
Heart Rate Variability.
HRV (Heart Rate Variability)
Low HRV = sympathetic dominance = chronic stress state
hs-CRP
High-sensitivity C-reactive protein — a blood marker for systemic inflammation.
HSAT
Home Sleep Apnea Test — a portable test that can catch many moderate-severe cases but can miss UARS, mild OSA, or more complex patterns.
Hydrogen (H₂) Breath
Elevated hydrogen indicates bacterial fermentation
Hydrogen Sulfide (H₂S) Breath
Third gas SIBO type — causes diarrhea, sulfur smell
hyperadrenergic POTS
A subtype of POTS with elevated norepinephrine, causing anxiety, tremor, and high blood pressure on standing in addition to tachycardia. May respond to clonidine, guanfacine, or beta-blockers. Diagnosed by standing norepinephrine levels >600 pg/mL.
hyperandrogenism
Excess male hormones (androgens) in women — causes acne, hirsutism, and hair loss. Key feature of PCOS. Can be clinical (visible symptoms) or biochemical (elevated testosterone/DHEA-S on blood tests).
hypoglossal nerve stimulation
An implanted therapy that stimulates tongue muscles during sleep to reduce airway collapse in selected people who cannot tolerate CPAP.
Hypoperfusion
Hypoperfusion can contribute to brain fog.
hypothyroid
Underactive thyroid — insufficient thyroid hormone production.
ICD-11
Burnout is now an official medical diagnosis.
IGeneX Panel
Specialty Lyme testing — detects more strains
IL-1
Interleukin-1β — a pro-inflammatory cytokine that reduces neurogenesis and disrupts sleep architecture. One of the key drivers of sickness behaviour and brain fog.
IL-6
Interleukin-6 — a pro-inflammatory cytokine. Elevated in Long COVID, chronic stress, and autoimmune conditions. Directly impairs hippocampal function (memory) and prefrontal cortex (focus).
IL-6 (Interleukin-6)
Pro-inflammatory cytokine elevated in Long COVID and chronic inflammation
IMO
Intestinal Methanogen Overgrowth. Methane-producing archaea in the gut, formerly called methane-dominant SIBO. Causes constipation and responds differently to treatment than hydrogen SIBO.
Insulin resistance
A state in which tissues respond poorly to insulin, often preceding type 2 diabetes and affecting brain-energy regulation too.
intestinal permeability
Often called 'leaky gut' — when the gut lining develops gaps, allowing bacteria, toxins, and undigested food particles to enter the bloodstream and trigger immune activation.
ISO
Intestinal Sulfide Overproduction. Hydrogen sulfide-producing bacteria in the gut. Associated with diarrhea and rotten egg odor. The newest recognized SIBO subtype.
k-means clustering
A machine learning algorithm that groups similar data points together. Applied to Long COVID symptoms, it identified 6 distinct phenotypes: sleep-dominant (58% PASC), triple-mix (80%), mental-dominant (66%), asymptomatic (1%), physical-dominant (51%), and physical+mental (85%).
Keto
Keto can contribute to brain fog, but the story pattern, nearby overlaps, and objective testing determine whether it is a meaningful driver.
ketogenic
A very-low-carbohydrate, high-fat diet that shifts the brain's fuel source from glucose to ketones.
Kidney
Kidney can contribute to brain fog.
L-theanine
An amino acid found in tea that can soften stress reactivity and promote calm attention without strong sedation in some people.
Lactulose breath test
A diagnostic test for SIBO. Patient drinks lactulose solution, then breath samples are collected over 2-3 hours measuring hydrogen and methane. Positive if hydrogen rises >20ppm in 90 minutes.
LDN
Low-dose naltrexone — naltrexone at 1–4.5 mg (vs. standard 50 mg) modulates the immune system and reduces neuroinflammation. Used off-label for Long COVID, fibromyalgia, and autoimmune brain fog. Requires prescription.
leaky gut
The colloquial term for increased intestinal permeability — gaps in the gut lining that allow inflammatory molecules into the bloodstream, triggering immune activation and brain fog.
Levine Protocol
Also called the CHOP protocol. A structured recumbent exercise program proven effective for POTS (86% adolescent remission rate). Starts with horizontal exercises (rowing, swimming, recumbent bike) and progressively adds upright training over 3-6 months.
LH
Luteinizing Hormone.
LH (Luteinizing Hormone)
Pituitary hormone — high LH with low T = primary hypogonadism
lipopolysaccharide
Bacterial fragments (endotoxins) from gram-negative gut bacteria. When they leak through a damaged gut barrier into the bloodstream, they trigger a powerful inflammatory response that reaches the brain.
Long COVID / ME/CFS
Long COVID / ME/CFS can contribute to brain fog.
Low-Dose Naltrexone
Naltrexone used at much lower doses than addiction treatment, usually around 0.5 to 4.5 mg, as an off-label immune and neuroinflammation modulator.
LPS
Lipopolysaccharides — bacterial endotoxins that trigger inflammation when they leak from the gut into the bloodstream through a damaged intestinal barrier.
LPS Antibodies
Indicates bacterial endotoxin translocation from gut
Lupus
Lupus can contribute to brain fog.
luteal phase
The second half of the menstrual cycle, from ovulation to the start of menstruation (roughly days 15-28). Progesterone peaks during this phase. PMDD symptoms occur during the luteal phase and resolve within a few days of menstruation.
Lyme
Lyme is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
mandibular advancement device
An oral appliance that pulls the lower jaw forward to keep the airway more open, often used for mild to moderate obstructive sleep apnea.
MARCoNS
Multiple Antibiotic Resistant Coagulase Negative Staphylococci — a nasal biofilm infection proposed in CIRS protocols as a contributing factor to chronic inflammation. Controversial in mainstream medicine; testing and treatment primarily used in CIRS-specialized practices.
MASLD
Metabolic dysfunction-associated steatotic liver disease (formerly NAFLD) — fatty liver disease driven by metabolic syndrome. Can cause fatigue, brain fog, and cognitive impairment through inflammation, insulin resistance, and impaired detoxification.
mast cell
Immune cells that release histamine and other chemicals during allergic and inflammatory reactions.
MBSR
Mindfulness-Based Stress Reduction, an 8-week structured program that combines meditation, body awareness, and gentle movement.
MCAS
Mast cell activation syndrome — mast cells release excessive histamine and other mediators, causing brain fog, flushing, hives, GI symptoms, and reactions to foods/chemicals.
MCV
Mean Corpuscular Volume.
MCV (Mean Corpuscular Volume)
Elevated MCV suggests B12/folate deficiency or alcohol
Medication Review
Identify fog-causing meds: benzos, anticholinergics, etc.
Meds
Meds can contribute to brain fog.
Melatonin
A hormone involved in sleep timing. Evening light exposure can delay the normal melatonin rise that helps prepare the brain for sleep.
Melatonin / DLMO
Melatonin is a hormone involved in sleep timing. DLMO means dim-light melatonin onset and is a marker of circadian phase.
Menopause
Menopause can contribute to brain fog.
Mercury
Mercury is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
Mercury / Heavy Metal Toxicity
Mercury / Heavy Metal Toxicity can contribute to brain fog.
Metabolic vascular
Metabolic vascular is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
Methane (CH₄) Breath
Elevated methane = IMO (intestinal methanogen overgrowth)
methylmalonic
Methylmalonic acid (MMA) — a more sensitive marker for B12 deficiency than serum B12 alone. Elevated MMA indicates functional B12 deficiency even when serum B12 is borderline normal.
Methylmalonic acid
A functional vitamin B12 marker. Elevated methylmalonic acid can indicate tissue-level B12 deficiency even when serum B12 still looks normal.
methylmercury (MeHg)
The organic form of mercury found in fish and seafood.
microbiome
The community of trillions of bacteria, fungi, and viruses in your gut.
microglia
The brain's resident immune cells.
Microvascular damage
Damage to small blood vessels, relevant in diabetes and hypertension because the brain depends on intact microcirculation.
Migraine
Migraine can contribute to brain fog.
mitochondri
Mitochondria — the energy-producing structures inside every cell. When they malfunction (from inflammation, nutrient deficiency, or viral damage), cells can't produce enough ATP (energy). Result: fatigue and brain fog.
MMA
If 200-500 with symptoms, add methylmalonic acid.
MMA (Methylmalonic Acid)
Elevated MMA confirms B12 deficiency at tissue level
MMC
The Migrating Motor Complex - your gut's 'cleaning wave' that sweeps bacteria and debris from the small intestine. It only activates during fasting (90-120 min after eating). Frequent snacking prevents MMC firing, promoting bacterial overgrowth.
MMP-9
Matrix Metalloproteinase-9.
MMP-9 (Matrix Metalloproteinase-9)
Elevated in active mold exposure and inflammation
MOH
Medication overuse headache.
Mold
Microscopic fungi that grow in damp indoor environments and can worsen respiratory, allergic, and sometimes cognitive symptom patterns when exposure is ongoing.
MRI
Diffusion Tensor Imaging.
MS
MS can contribute to brain fog.
MSH
Melanocyte-Stimulating Hormone.
MSH (Melanocyte-Stimulating Hormone)
Low MSH = impaired immune regulation in CIRS
MSPSS
Multidimensional Scale of Perceived Social Support — a 12-item questionnaire measuring support from family, friends, and significant others. Ranked 13th predictor in Long COVID ML classification. Low social support = measurably worse outcomes. Treatable via social prescribing and support groups.
Mycotoxin
A toxin produced by some molds. Exposure concerns are most relevant in water-damaged environments and should be handled with careful, non-sensational wording.
myo-inositol
A naturally occurring compound that improves insulin sensitivity in PCOS. Dose: 2-4g/day. Meta-analyses show improvement in ovulation, hormone levels, and metabolic markers. Often combined with D-chiro-inositol (40:1 ratio).
N-Methylhistamine (24hr Urine)
Histamine metabolite — more stable than plasma histamine
N3 sleep
Deep slow-wave sleep, the stage most associated with feeling physically restored and mentally clearer the next day.
NAC
N-acetyl cysteine — an amino acid supplement that replenishes glutathione (the body's master antioxidant), reduces neuroinflammation, and helps break down mucus. Dose: 600–1,200 mg/day.
NAD
Nicotinamide adenine dinucleotide — a coenzyme essential for cellular energy production. Declines with age. NAD+ precursors (NR, NMN) are being studied for cognitive recovery, especially in Long COVID.
NAFLD/MASLD
Fatty liver disease in its older and newer naming systems.
NASA Lean Test
A simple orthostatic screening test: stand leaning against a wall (heels 6 inches from wall) for 10 minutes.
neurogenesis
The creation of new neurons, primarily in the hippocampus (memory centre).
Neuroglycopenic
Symptoms caused by the brain not getting enough usable glucose, such as confusion, slowed thinking, or difficulty concentrating.
neuroinflammation
Inflammation specifically in the brain and nervous system.
Neurological red flags
Neurological red flags can contribute to brain fog.
Neuropathic POTS
A subtype of POTS caused by damage to small fiber nerves controlling blood vessels. Often associated with autoimmune conditions. May respond to IVIG or other immunomodulatory treatments. Diagnosed via skin biopsy showing reduced nerve fiber density.
neuroplasticity
The brain's ability to rewire itself by forming new neural connections. Brain fog doesn't cause permanent damage — neuroplasticity means recovery is possible once the underlying cause is addressed.
Neuropsych Testing
Tests memory, attention, executive function, processing speed
Neuropsychological Evaluation
Gold standard for ADHD and cognitive dysfunction
neurosteroid
Steroid hormones produced in the brain that modulate neurotransmitter receptors, particularly GABA. Allopregnanolone is the key neurosteroid implicated in PMDD — abnormal sensitivity to its fluctuations may cause luteal-phase symptoms.
NF-kB
Nuclear factor kappa-B — a protein complex that controls inflammation gene expression. When chronically activated, it drives sustained neuroinflammation. Curcumin, omega-3s, and NAC help modulate it.
Nicotine
Nicotine can contribute to brain fog.
NLR
Neutrophil-to-Lymphocyte Ratio — a calculated inflammation marker from a standard CBC.
NLR (calculated)
Neutrophil-to-lymphocyte ratio derived from CBC differential as an inflammatory context signal.
NMN
Nicotinamide mononucleotide — a precursor to NAD+, the coenzyme essential for mitochondrial energy production. Emerging evidence for cognitive benefits, particularly in age-related and post-viral brain fog.
Non-stimulant medication
ADHD treatments such as atomoxetine, guanfacine XR, or clonidine XR that do not use stimulant mechanisms.
noradrenaline
Also called norepinephrine. The neurotransmitter that governs alertness. In hyperadrenergic POTS, excessive noradrenaline causes anxiety, tremor, and palpitations alongside orthostatic symptoms.
norepinephrine
A neurotransmitter and stress hormone that governs alertness and attention. Too little = foggy and fatigued. Too much = anxious and scattered. Cold exposure triggers a reliable spike.
Notification
A push alert, banner, vibration, or badge that pulls attention toward a device and can create interruption costs even when ignored.
NPH
Normal-pressure hydrocephalus.
NPSLE
Neuropsychiatric lupus.
Nrf2
A protein that activates your body's antioxidant defence genes. Sulforaphane (from broccoli sprouts), curcumin, and exercise activate Nrf2. It's one of the most powerful natural anti-inflammatory pathways.
NRT
Nicotine replacement therapy.
NSDR
Non-sleep deep rest. A guided rest practice used to downshift arousal without necessarily falling asleep.
Nutrient
Nutrient is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
ODI
Oxygen Desaturation Index — the number of times oxygen drops during sleep, which can matter even when people focus only on the AHI.
omega-3 confounding
A key challenge in mercury research: people with higher mercury levels typically eat more fish, which also provides neuroprotective omega-3 fatty acids. This makes it difficult to separate mercury's harmful effects from fish's beneficial effects in observational studies.
Omega-6:3 ratio
Fatty-acid balance marker used to contextualize inflammatory dietary patterns.
organophosphate
A class of pesticides that inhibit acetylcholinesterase, disrupting nervous system function. Common in agriculture. Acute exposure causes cholinergic crisis; chronic low-level exposure is linked to cognitive impairment, neuropsychiatric symptoms, and brain fog.
Orthostatic BP Measurement
Blood pressure change from lying to standing
orthostatic intolerance
Symptoms triggered by upright posture: dizziness, brain fog, palpitations, nausea, weakness. Umbrella term that includes POTS, orthostatic hypotension, and other conditions where the body cannot properly regulate blood flow when vertical.
Orthostatic Vitals
Combined HR and BP measurements with position
OSA
Obstructive sleep apnea, a common metabolic-syndrome overlap that independently causes cognitive impairment.
P300
An event-related potential (brain response) occurring approximately 300 milliseconds after a stimulus, reflecting cognitive processing speed and attention allocation. Significantly delayed in brain fog patients (Fabio 2024: F(1,39)=16.11, p<0.01), meaning the brain takes measurably longer to classify relevant information. Persists at 8 months post-infection.
P300 ERP
Event-related potential index of processing speed and attention timing.
Pain
Pain is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
Pain Scales
Standardized pain assessment tools
parasympathetic
The 'rest-and-digest' branch of the autonomic nervous system, controlled primarily by the vagus nerve. Activating it reduces heart rate, lowers inflammation, and supports cognitive recovery.
PASC
Post-Acute Sequelae of SARS-CoV-2 infection — the formal medical term for Long COVID.
PCFS
Post-COVID Functional Scale — a validated measure of functional status after COVID-19, grading ability to perform daily activities. Ranked #1 predictor (importance 0.0766) in ML classification — how much you can do is the strongest signal of Long COVID severity.
PCOS
PCOS can contribute to brain fog.
Pcs
Pcs can contribute to brain fog.
PEA
Palmitoylethanolamide — a naturally occurring fatty acid with anti-inflammatory and neuroprotective properties. Works by calming overactive microglia. Dose: 600–1,200 mg/day. Well-tolerated with few interactions.
PEM
Post-exertional malaise — the hallmark symptom of ME/CFS and many Long COVID cases.
Pesticides
Pesticides can contribute to brain fog.
PHES
Psychometric Hepatic Encephalopathy Score, a validated paper-based test battery for covert hepatic encephalopathy.
Phosphatidylserine
A phospholipid supplement sometimes used to blunt stress reactivity, especially when the problem feels like a high-alert response to mental or exercise load.
photobiomodulation
Using specific wavelengths of red or near-infrared light to stimulate mitochondrial function in brain cells. Transcranial devices (worn on the head) have shown cognitive improvements in Long COVID trials.
PHQ-9
Depression screening — overlap with brain fog symptoms
Plasma Histamine
Direct histamine measurement — elevated causes brain fog, flushing
PLR
Platelet-to-Lymphocyte Ratio — a blood inflammation marker calculated from CBC.
PLR (calculated)
Platelet-to-lymphocyte ratio derived from CBC values used as an adjunct inflammatory/vascular signal.
PM2.5 Sensor
Fine particulate matter — affects brain inflammation
PMDD
PMDD can contribute to brain fog.
PNE
Pain neuroscience education.
polysomno
Polysomnography — an overnight sleep study that monitors brain waves, breathing, oxygen levels, and muscle activity. The gold standard for diagnosing sleep apnea, UARS, and other sleep disorders.
polysomnography
An overnight in-lab sleep study that monitors brain waves, breathing, oxygen levels, heart rhythm, and body position.
Polysomnography (Sleep Study)
Gold standard for sleep apnea diagnosis
Post surgical
Post surgical can contribute to brain fog.
Postpartum
Postpartum can contribute to brain fog.
Postprandial
Occurring after a meal. Postprandial fog means symptoms that reliably worsen after eating.
Pots
Pots is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
prefrontal cortex
The front region of your brain responsible for planning, decision-making, working memory, and focus.
Pregnancy
Pregnancy can contribute to brain fog.
Pregnenolone
A steroid precursor sometimes marketed for memory, mood, or hormonal support. It is hormone-adjacent and should be treated more cautiously than a standard over-the-counter supplement.
Problematic smartphone use
A pattern of phone use that has become compulsive, impairing, or hard to control despite clear downsides.
Progesterone
Low progesterone linked to anxiety, insomnia, fog
Prokinetic
A medication or supplement that stimulates gut motility (movement). Used in SIBO maintenance to keep the MMC firing and prevent recurrence. Examples: low-dose erythromycin, prucalopride, ginger.
Prolactin
Elevated prolactin suppresses testosterone
PROMIS cognitive
Patient-Reported Outcomes Measurement Information System Cognitive Function — a validated questionnaire measuring subjective cognitive complaints. Ranked 5th predictor (importance 0.0626) in Long COVID ML classification. Gives doctors a validated number instead of 'I feel foggy.'
Prostaglandin D2
Mast cell mediator — elevated in MCAS and allergic reactions
Provoked Challenge Test
DMSA/DMPS chelation challenge — controversial but used
PSG
Sleep Study.
PSQI
Pittsburgh Sleep Quality Index — a standard questionnaire that helps quantify how restorative or disrupted sleep has felt over the last month.
PSS-10
Perceived Stress Scale — a 10-item validated questionnaire measuring stress levels. Ranked 11th predictor (importance 0.0421) in Long COVID ML classification. Score 14-26 = moderate stress, 27+ = high. High stress is as predictive as clinical measures like headache.
Psychiatric
Psychiatric can contribute to brain fog.
PTLDS
PTLDS is a relevant clinical term in this differential and should be clarified before interpreting this cause.
PTSD
PTSD can contribute to brain fog.
pyrethroid
A class of synthetic pesticides derived from chrysanthemum flowers. Used in household insect sprays, pet treatments, and agriculture. Generally considered safer than organophosphates but can cause neurological symptoms at high exposure.
QbTest
Computer-based attention and impulsivity testing
QEEG
Quantitative electroencephalography. A processed EEG map that compares brain-wave patterns against a normative database and is sometimes used before neurofeedback.
quantitative EEG
Digital analysis of brain electrical activity patterns, breaking the raw EEG signal into frequency bands (delta, theta, alpha, beta, gamma) to quantify brain function. Research shows measurable differences in brain fog patients vs healthy controls: reduced alpha (8-13 Hz) and increased theta (4-7 Hz). Currently research-grade only — not yet a standard clinical diagnostic for brain fog.
Quantitative EEG (qEEG)
Electrophysiology-based mapping sometimes used in specialist cognitive evaluation.
random forest
A machine learning algorithm that builds multiple decision trees and combines their predictions. It is commonly used to rank variables, classify groups, or estimate risk from many overlapping features.
Rapid Alternating Movement
Bedside coordination screen assessing dysdiadochokinesia and cerebellar-motor timing.
RBC
Glutathione.
RBC Magnesium
Intracellular magnesium — serum levels miss deficiency
RDI
Respiratory Disturbance Index — a broader count that can include arousal-based breathing events beyond classic apneas and hypopneas.
Reactive hypoglycemia
A post-meal spike-crash pattern where symptoms such as shakiness, irritability, and brain fog appear 1 to 4 hours after eating.
REM sleep
Rapid eye movement sleep, a stage tied to memory, dreaming, and emotional processing. Repeated interruptions here can leave mornings fuzzy.
REM-predominant OSA
A pattern where breathing events cluster more heavily in REM sleep, which can make a person feel foggy even when part of the night looked milder.
Remediation
The process of correcting the moisture problem and safely removing or containing damaged material so mold growth does not keep recurring.
RERA
Respiratory effort-related arousal, a subtle sleep-breathing event that can fragment sleep without classic severe apnea.
RERAs
Respiratory effort-related arousals — brief awakenings caused by increased breathing effort that don't meet the criteria for apnea. Cause fragmented sleep and daytime fog. Many sleep labs don't score them unless asked.
Reverse T3
An inactive thyroid hormone metabolite sometimes checked when thyroid symptoms persist despite a basic thyroid panel that looks acceptable.
Rheumatoid Factor
Elevated in rheumatoid arthritis and other autoimmune conditions
Rhodiola rosea
An adaptogenic herb studied mainly for stress-related fatigue and strain. It is better framed as a fatigue-support adjunct than as proof of “adrenal fatigue.”
Rifaximin
A non-absorbable antibiotic that stays in the gut and targets small intestinal bacteria. The first-line prescription treatment for SIBO, especially hydrogen-dominant cases.
Romberg Balance
Patient-facing Romberg balance exam route covering the bedside balance screen often recommended in neurological or vestibular context.
Romberg Balance (eyes closed)
Balance and proprioception screen used in vestibular and neurological exam context.
Rotterdam Criteria
The international diagnostic standard for PCOS requiring 2 of 3 features: oligo/anovulation, clinical or biochemical hyperandrogenism, and polycystic ovarian morphology on ultrasound. Other causes must be excluded first.
Salivary cortisol
A saliva-based cortisol measurement often used in stress research and late-night Cushing screening because it is easy to collect at home.
SCFA
Short-chain fatty acids — metabolites produced by gut bacteria from dietary fibre (butyrate, propionate, acetate). They strengthen the gut barrier, reduce inflammation, and support brain function.
Screen time
Total time spent using phones, tablets, computers, or other screens. On this page, the more useful clue is not the raw total alone but whether high-screen days map to worse symptoms.
Screen Time Audit
Digital wellness assessment
SDMT
The Symbol Digit Modalities Test.
Sedentary
Sedentary can contribute to brain fog.
serotonin
A neurotransmitter regulating mood, sleep, and gut function.
Serum Osmolality
Dehydration marker — affects cognitive performance
Serum Potassium
Abnormal potassium affects nerve and muscle function
Serum Sodium
Core electrolyte marker that may affect fatigue, cognition, and autonomic symptoms.
Serum Tryptase
Mast cell activation marker — elevated in MCAS
SGLT2 inhibitor
A class of drugs that lowers glucose by increasing urinary glucose excretion and also protects the heart and kidneys.
SGLT2i
Sodium-glucose cotransporter 2 inhibitors — diabetes medications that cause glucose excretion in urine. Beyond glucose control, they show cardiovascular and kidney protection. Examples: empagliflozin, dapagliflozin.
SHBG
Sex Hormone Binding Globulin.
SHBG (Sex Hormone Binding Globulin)
High SHBG = less free testosterone available
Shoemaker Protocol
A functional-medicine protocol for chronic inflammatory response syndrome that starts with exposure removal, then may use binders, lab-guided corrections, and other specialty steps. It is not established mainstream care.
short-chain fatty acid
Metabolites (butyrate, propionate, acetate) produced when gut bacteria ferment fibre. They strengthen the gut lining, reduce systemic inflammation, and directly support neuron health.
SIBO
Small intestinal bacterial overgrowth — bacteria that should live in the large intestine colonise the small intestine, causing bloating, malabsorption, and brain fog via the gut-brain axis.
SIBO Breath Test
Screens for small intestinal bacterial overgrowth
SIFO
of patients with unexplained GI symptoms had small intestinal fungal overgrowth.
sIgA (Secretory IgA)
Gut immune function — low in chronic stress, dysbiosis
SII
Systemic Immune-Inflammation Index — a composite blood marker calculated from CBC that tracks overall immune activation.
SIMO
Small Intestinal Microbial Overgrowth. A newer umbrella term for SIBO that includes bacteria, archaea (methanogens), and other microbes, not just bacteria.
Sleep
Sleep is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
Sleep apnea
Sleep apnea is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
Sleep Latency Testing
Time to fall asleep — <8 min indicates severe sleep deprivation
Sleep Study (PSG)
Overnight polysomnography explainer framed around the patient-facing 'sleep study' language most people actually search.
Social
Social can contribute to brain fog.
Social jetlag
The mismatch between your biological clock and your social schedule, often seen when weekend wake times drift far later than workday wake times.
social prescribing
A healthcare approach where clinicians prescribe social activities (support groups, community programs, peer connections) alongside medical treatments.
Somatopause
The age-related decline in growth hormone and IGF-1 signaling that may overlap with reduced recovery capacity, muscle maintenance, and resilience.
SPECT Scan
Brain perfusion imaging — shows hypoperfusion areas
SpO2
Peripheral oxygen saturation — the oxygen reading used on sleep tests and pulse oximetry to show how low breathing events are driving the blood oxygen level.
statin
Cholesterol-lowering medications. Some people report cognitive side effects (memory problems, confusion). Lipophilic statins (atorvastatin, simvastatin) cross the blood-brain barrier more readily than hydrophilic ones.
Stimulant medication
First-line ADHD medications such as methylphenidate and amphetamine formulations.
STOP-BANG
A rapid 8-item screening questionnaire for obstructive sleep apnea risk.
Sugar
Sugar is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
sympathetic
The 'fight-or-flight' branch of the autonomic nervous system. Chronic sympathetic dominance (from ongoing stress) drives cortisol elevation, inflammation, and brain fog.
symptom cluster
A group of symptoms that tend to occur together, identified through clinical pattern recognition or statistical analysis. Cluster language is useful when the same diagnosis can show up in different dominant patterns.
Symptom Tracking (2+ Cycles)
Map fog timing to cycle phases for hormonal patterns
Telepressure
The felt pressure to respond quickly to messages and digital communication, even when the device is technically silent.
Testosterone
Primary male hormone — affects cognition, mood, energy
TGF-β1
Mold/CIRS marker — elevated in biotoxin illness
theta rhythm
Brain waves in the 4-7 Hz frequency range, associated with drowsiness, cognitive slowing, and reduced information processing efficiency. Elevated theta power in brain fog patients correlates with subjective fatigue severity (Fabio 2024). Think of theta as your brain's 'buffering' indicator — when it's elevated, information processing is sluggish.
Thyroglobulin Antibodies
Second marker for autoimmune thyroid disease
Thyroid
Thyroid is a nearby overlapping cause that is often worth ruling out when the story pattern is similar.
TIBC
Total iron-binding capacity — measures how much iron your blood can carry. High TIBC with low ferritin confirms iron deficiency, even when haemoglobin looks normal.
tilt table test
The gold standard diagnostic test for POTS.
Time blindness
Difficulty sensing how long tasks take, how much time has passed, or how close a deadline really is.
TNF
Tumour necrosis factor-alpha — a pro-inflammatory cytokine. Elevated in chronic inflammation, obesity, and autoimmune disease. Impairs synaptic plasticity (the brain's ability to form new connections).
TNF-α (Tumor Necrosis Factor Alpha)
Key inflammatory cytokine — elevated crosses blood-brain barrier
Total + Free Testosterone
This panel should be used selectively.
Total Testosterone
Low T impairs verbal memory and processing speed
tPBM
Transcranial photobiomodulation: red or near-infrared light delivered to the head to target brain tissue rather than skin or muscle.
TPO
Thyroid peroxidase antibodies — elevated levels indicate Hashimoto's thyroiditis (autoimmune thyroid disease), one of the most commonly missed causes of brain fog.
TPO Antibodies
Elevated in Hashimoto's thyroiditis — autoimmune thyroid attack
transcranial
Delivered through the skull to the brain — refers to non-invasive therapies like transcranial photobiomodulation (light) or transcranial direct current stimulation (electrical) that target brain tissue from outside.
Transcranial Doppler
Measures cerebral blood flow — detects hypoperfusion
Trauma
Trauma can contribute to brain fog.
Trio-Smart (3-Gas Breath Test)
Advanced at-home breath test measuring hydrogen, methane, AND hydrogen sulfide to distinguish SIBO, IMO, and ISO subtypes.
TSH
Thyroid-stimulating hormone — the standard thyroid screening test.
TSH (Thyroid Stimulating Hormone)
Primary thyroid screening marker.
TSH + B12 + Ferritin
This grouped panel is useful when the story could reflect thyroid slowdown, iron depletion, or B12-related cognitive symptoms and you want one first-pass conversation instead of three disconnected requests.
TSH + Free T3 + Free T4
This panel helps frame whether the story fits thyroid slowdown, conversion issues, or a closer competitor cause before you default to broad lifestyle explanations.
tTG-IgA (Celiac)
Patient-facing celiac serology explainer route focused on the test wording users actually bring from clinician visits.
tTG-IgA (Tissue Transglutaminase)
Celiac disease screening — gluten triggers neuroinflammation
Tyrosine
An amino acid used as a precursor in catecholamine synthesis. It is one reason protein is discussed in the ADHD food section.
UARS
Upper Airway Resistance Syndrome, a sleep-related breathing problem that can mimic ADHD-like fatigue and brain fog.
UARS/RERA Scoring
Detects subtle upper airway resistance missed by standard AHI
UK
UK is a relevant clinical term in this differential and should be clarified before interpreting this cause.
Upright MRI
Detects cervical instability missed by supine MRI
Uremic toxins
Waste molecules that accumulate when kidney function declines and can contribute to cognitive symptoms.
URGENT
URGENT is a relevant clinical term in this differential and should be clarified before interpreting this cause.
Urine Mercury
Longer-term mercury exposure marker
Urine Organic Acids Test
Detects metabolic dysfunction, mitochondrial issues, toxins
vagal
Relating to the vagus nerve — the main nerve of the parasympathetic nervous system. Vagal tone refers to the activity level of this nerve. Higher vagal tone = better stress resilience and lower inflammation.
vagus nerve
The longest cranial nerve, connecting the brain to the gut, heart, and lungs. It controls the parasympathetic (rest-and-digest) nervous system. Stimulating it (cold exposure, deep breathing, gargling) reduces inflammation.
VCS Test (Visual Contrast Sensitivity)
Free screening for mold/CIRS — reduced contrast sensitivity
vestibular
Relating to the inner ear balance system.
Vestibular Assessment
Tests balance system — dysfunction causes dizziness, fog
VIP
Vasoactive intestinal peptide — a neuropeptide with anti-inflammatory and regulatory functions. Used as a nasal spray in some CIRS protocols for patients who don't fully respond to other treatments. Requires prescription and monitoring.
VIP nasal spray
Vasoactive intestinal peptide delivered intranasally. This is a late-step treatment in some Shoemaker-style protocols and is not a standard mainstream mold therapy.
Vitamin B12
Patient-facing vitamin B12 explainer route, useful when a story or clinician uses plain language instead of the active-B12 variant.
Vitamin D
Vitamin D can contribute to brain fog.
Vitamin D (25-OH)
Patient-facing vitamin D explainer route matching the common 25-OH wording used in lab and search language.
VOC
Volatile organic compounds. Chemicals released from paints, cleaners, fragrances, and building materials that can worsen indoor air quality.
VOMS
vestibular screening.
Welchol
Brand name for colesevelam, another bile-acid sequestrant sometimes used as an alternative binder when cholestyramine is not tolerated.
Western Blot (Lyme)
Lyme confirmatory test — follows positive ELISA
Working memory
The ability to hold information in mind long enough to use it. In ADHD this often feels brittle or inconsistent.
WURS
The Wender Utah Rating Scale, often used to help reconstruct childhood ADHD symptoms.
Yoga Nidra
A guided resting meditation sometimes used as an NSDR-style practice to reduce arousal and support recovery.
zonulin
A protein that regulates the tight junctions between gut lining cells. Gluten and certain bacteria trigger zonulin release, opening gaps in the gut wall (intestinal permeability) and allowing inflammatory molecules into the bloodstream.
Note on Clinical Language
The Field Guide uses precise medical terminology to ensure accuracy. If you encounter a term not listed here, please check our editorial methodology or consult your healthcare provider for clarification.
Related Causes
Glossary readers often need direct examples tied to technical terms.