Brain Fog: Causes, Related Tests & What Helps

What Is Brain Fog?

Brain fog isn't a medical diagnosis itself: it's a cluster of symptoms including poor concentration, forgetfulness, and a feeling of mental fatigue or confusion. It can stem from many underlying causes.

Common Causes

• Poor sleep or sleep disorders
• Chronic stress and anxiety
• Hypothyroidism
• Vitamin B12 or vitamin D deficiency
• Anaemia (low iron)
• Blood sugar imbalances
• Post-viral fatigue (including post-COVID)
• Certain medications

Tests Your Doctor May Order

What Helps

• Prioritising 7–9 hours of quality sleep
• Treating any underlying vitamin deficiency or thyroid issue
• Regular physical activity
• Managing stress with relaxation techniques
• Limiting alcohol and reducing screen time before bed
• Staying hydrated and eating balanced meals

Frequently Asked Questions

References

Sources cited on this page. PubMed links open the original abstract.

1. Ocon AJ. Caught in the thickness of brain fog: exploring the cognitive symptoms of myalgic encephalomyelitis/chronic fatigue syndrome. Front Physiol. 2013;4:63. PMID 23576989 · doi:10.3389/fphys.2013.00063

The Biology Behind Brain Fog: What Is Actually Happening?

Brain fog is not a recognised clinical diagnosis but a symptom cluster – cognitive slowing, word-finding difficulty, poor concentration, and memory lapses – that patients use to describe a measurable impairment in mental processing. Neurologically, brain fog reflects disrupted prefrontal cortex function, often driven by systemic inflammation, altered neurotransmitter availability, mitochondrial dysfunction, or hormonal dysregulation. Understanding which pathway is active in a given patient is what blood tests help determine.¹

One of the most important and reversible causes is thyroid dysfunction. Hypothyroidism reduces the availability of thyroid hormones needed for neuronal metabolism; even subclinical hypothyroidism (raised TSH with normal free T4) produces measurable cognitive impairment in some individuals. A 2019 meta-analysis in Thyroid confirmed that cognitive complaints including brain fog significantly improve in hypothyroid patients after levothyroxine treatment is optimised, particularly when TSH is normalised to the lower half of the reference range.²

Blood Tests That Identify Treatable Causes

A systematic approach to brain fog starts with a core panel targeting the most common reversible causes. Thyroid function (TSH and free T4) identifies hypothyroidism and hyperthyroidism – both impair cognition by different mechanisms. Full blood count detects anaemia: iron-deficiency anaemia reduces cerebral oxygen delivery; B12-deficiency anaemia causes demyelination of neural pathways that presents as cognitive slowing, peripheral neuropathy, and mood change. Serum ferritin is the most sensitive marker of depleted iron stores, often falling before haemoglobin drops.

Fasting glucose and HbA1c identify diabetes and insulin resistance, both of which impair cognitive function through glycation of neural proteins and chronic low-grade inflammation. Vitamin D (25-hydroxyvitamin D) deficiency has been associated with cognitive complaints in multiple observational studies; the mechanism involves vitamin D receptors expressed in hippocampal neurons involved in memory consolidation. CRP and ESR identify systemic inflammation from autoimmune conditions – including lupus, coeliac disease (checked via tissue transglutaminase IgA), and inflammatory bowel disease – that cause brain fog through neuroinflammatory pathways rather than direct CNS involvement.³

Post-Viral Brain Fog and Long COVID

The COVID-19 pandemic placed brain fog at the centre of medical attention. Long COVID cognitive symptoms – affecting an estimated 20–30% of those with persistent post-acute sequelae – appear to involve microglial activation, endothelial inflammation, and persistence of viral antigen in the central nervous system. Standard blood panels are typically unremarkable in long COVID brain fog, which is diagnostically frustrating but clinically important: it means a normal CRP, normal thyroid, and normal B12 do not rule out neurological post-viral sequelae.

D-dimer elevation in some long COVID patients points to ongoing microthrombus formation in cerebral vasculature as a contributing mechanism. Research continues to identify specific biomarkers – including elevated GFAP (glial fibrillary acidic protein), a marker of astrocyte injury measurable in blood – but these remain research tools rather than standard clinical tests. For now, the practical approach is to rule out treatable causes with a systematic blood panel while acknowledging that a normal result does not invalidate the symptom.

Why brain fog is hard to pin down

Brain fog is a description, not a diagnosis. Nobody can measure it directly, and there is no blood test for it. What people usually mean is some combination of slowed thinking, difficulty holding several things in mind at once, losing words mid-sentence, and having to reread the same paragraph. Because those features overlap with normal tiredness, the useful question is not whether the feeling is real but whether something correctable is driving it.

Two patterns help separate the causes. The first is timing: fog that is worst on waking and improves through the day points towards sleep quality, including sleep apnoea, whereas fog that builds through the afternoon more often tracks blood sugar, dehydration or medication timing. The second is company: fog accompanied by cold intolerance, constipation and weight gain suggests thyroid; with pins and needles or a sore tongue, B12; with joint pain and rashes, an autoimmune cause.

Medications that commonly contribute

Drug side effects are among the most reversible causes and among the most frequently missed, partly because people do not associate a medicine they have taken for years with a symptom that started recently. Dose changes, added medicines and declining kidney function can all tip a previously tolerated drug into causing symptoms.

• Sedating antihistamines such as chlorphenamine and promethazine, including those sold for sleep.
• Anticholinergic medicines for bladder urgency, and some older antidepressants; effects add up when several are taken together.
• Benzodiazepines and z-drugs, whose daytime carryover is often mistaken for the insomnia they were prescribed for.
• Opioid painkillers, particularly in the first weeks and after any dose increase.
• Gabapentin and pregabalin, which commonly cause word-finding difficulty at higher doses.

What to track before your appointment

A two-week diary is more useful than trying to recall the pattern in the consulting room. Note roughly how many hours you slept and whether you woke unrefreshed, when the fog was worst, what you had eaten, and any alcohol. If you snore or have been told you stop breathing at night, record that too. Bringing a fortnight of that detail frequently shortens the path to the right test, because it converts a vague complaint into a pattern a clinician can act on.