Blood Tests for Brain Fog: Common Causes to Check

Brain fog is not a medical diagnosis, but most people know exactly what it means: struggling to concentrate, losing your train of thought, forgetting words or feeling as though you are thinking through cotton wool. It is common after illness, during perimenopause, with poor sleep and under stress, and it can also be a sign of something a blood test can find, such as low vitamin B12 or an underactive thyroid.

This guide covers the blood tests most worth checking for brain fog, what each can and cannot show, which panel to choose and the warning signs that need urgent help rather than a routine test. Your result is a starting point for a conversation with a practitioner or your GP, not a diagnosis.

Test it in Chiswick: our Tired All The Time blood test costs £159, with your result explained in person by a practitioner. No GP referral needed. Book this test or call 020 4515 0000.

Quick answer

The key blood tests for brain fog are vitamin B12 and folate, thyroid function (TSH and free T4), ferritin with a full blood count for iron and anaemia, vitamin D and HbA1c for blood sugar. The NHS lists problems with memory, understanding and judgement as symptoms of B12 or folate deficiency, and trouble concentrating as a symptom of an underactive thyroid. Hormone tests may help some women under 45 with perimenopause symptoms. Sudden confusion is different: it needs A&E or 999, not a blood test.

Blood tests that can help

Vitamin B12 and folate

The NHS lists problems with memory, understanding and judgement among the symptoms of vitamin B12 or folate deficiency, and these can appear before anaemia shows on a blood count. Low B12 can also cause pins and needles, numbness, balance problems and low mood. Our Vitamin B12 test costs £49, and Folate (Folic Acid) also costs £49. Read more in our guides to vitamin B12 levels and folate levels.

Thyroid function

An underactive thyroid tends to develop slowly. The NHS lists trouble concentrating or thinking clearly, tiredness and low mood among its symptoms, and the British Thyroid Foundation adds memory problems and slow thoughts. Our Thyroid test (£59) measures TSH, free T3, free T4 and two thyroid antibodies. A home finger-prick thyroid kit is also available, ask us when you enquire, and see home kits. Our thyroid results guide explains the numbers.

Ferritin and full blood count

Iron deficiency anaemia causes tiredness, lack of energy, headaches and breathlessness, which can all make thinking harder. A Ferritin test (£49) shows your iron stores, and a Full Blood Count (£59) checks haemoglobin and the size of your red cells, which can hint at low iron or low B12.

Vitamin D

There is no strong evidence that low vitamin D causes brain fog on its own, but it is common in the UK because sunlight is not strong enough from about October to March. It is often checked as part of a wider tiredness screen. A Vitamin D test costs £59, and NHS advice is to consider 10 micrograms a day in autumn and winter.

HbA1c (blood sugar)

Type 2 diabetes can cause tiredness and blurred vision and often develops without obvious symptoms. An HbA1c test (£59) shows your average blood sugar over the past two to three months. If you already have diabetes, low blood sugar is a known cause of sudden confusion and needs treating straight away.

Perimenopause hormones

The NHS notes that problems with memory or concentration can happen during perimenopause, often alongside changes to periods, hot flushes and poor sleep. NICE advises that perimenopause in women aged 45 and over is usually recognised from symptoms without blood tests. Hormone tests, such as FSH, are mainly useful between 40 and 45, or under 40 when early menopause is suspected. Our Hormone Health Check (£69) includes FSH, LH and oestradiol. See our guide to FSH levels.

Long COVID and post-viral brain fog

The NHS lists problems with memory and concentration, also called brain fog, as a common symptom of long COVID, which describes symptoms lasting more than 12 weeks after infection. There is no blood test that diagnoses long COVID, but GPs often use blood tests to look for other causes. Our Post-Viral Recovery Screen (£139) checks blood count, iron, thyroid, vitamin D, inflammation and several heart and muscle markers.

Which test should I book?

• A focused start: Tired All The Time, £159. Covers B12, folate, a full thyroid profile, iron studies with ferritin, vitamin D, HbA1c, a full blood count, kidney function and CRP in one sample. The best single panel for most people with brain fog.
• A broader picture: Vitamin B Profile, £329. Looks more closely at B vitamins, including active B12, B1 and B6, which suits people with tingling, a restrictive diet or borderline B12 results.
• The fullest view: Advanced GP3, £339. A comprehensive annual-style screen covering vitamins, thyroid, sex hormones, blood sugar, heart, liver and kidney markers.

What blood tests cannot tell you

Many common causes of brain fog do not show up in blood at all. Poor or broken sleep, stress, anxiety, low mood, alcohol, some prescribed medicines and long working hours can all affect concentration. Normal results do not mean your symptoms are not real. They help rule out treatable causes so you and your GP can focus on what is left.

Blood tests also cannot diagnose dementia or long COVID. Memory problems that are getting steadily worse need a proper assessment with your GP, who may refer you to a memory service.

When to see a GP urgently

Go to A&E or call 999 if someone suddenly becomes confused. Common causes of sudden confusion include infection, stroke, low blood sugar in people with diabetes, head injury and some medicines.

Call 999 straight away if you notice signs of a stroke. Remember FAST:

• Face: the face may have dropped on one side, or the person cannot smile.
• Arms: the person cannot lift both arms and keep them there.
• Speech: speech may be slurred or garbled, or the person cannot talk.
• Time: call 999 immediately, even if the symptoms stop.

Book a GP appointment if you or a relative have gradually become more forgetful or confused, or if brain fog comes with numbness, pins and needles, balance problems, unexplained weight loss or symptoms that are getting worse.

Frequently asked questions

Can low B12 cause brain fog?

Yes. The NHS lists problems with memory, understanding and judgement as symptoms of B12 deficiency, and they can occur without anaemia. B12 is one of the first tests to check.

Can thyroid problems cause brain fog?

An underactive thyroid commonly causes trouble concentrating, memory problems and tiredness. A thyroid blood test is simple and is usually part of any brain fog check.

Is brain fog a sign of perimenopause?

It can be. Problems with memory or concentration are recognised perimenopause symptoms and may feel worse with poor sleep. If you are over 45, a GP can usually recognise perimenopause from your symptoms without blood tests.

Can a blood test diagnose long COVID?

No. Long COVID is diagnosed from your symptoms and history. Blood tests are used to rule out other causes, such as anaemia, thyroid problems or low vitamins.

What if all my blood tests are normal?

That is common and still useful. Your practitioner can talk through sleep, stress, mood and medicines, and your GP can decide whether you need other tests or a referral.

Brain fog blood testing in Chiswick

Omnia Lifestyle is at 8 to 9 Bedford Corner, The Avenue, Chiswick, London W4 1LZ, a short walk from Turnham Green station (District line; Piccadilly line trains stop there early morning and late evening only) and on the 94, E3 and 272 bus routes. Limited parking is available on request when you book. Every clinic test price includes your sample taken at the clinic, analysis at the Randox accredited laboratory, a review of your result with a practitioner and a GP feedback report on request. No GP referral is needed and most results are back within a few working days.

Book the Tired All The Time panel, look at the menopause blood tests, or compare every blood test and price. You may also find our guides to vitamin B12 levels and thyroid test results helpful.

This article is general information, not medical advice. Discuss your results with your GP or practitioner.