Brain Fog

"Brain fog" is not a medical diagnosis, which is why people are sometimes told it is nothing. It is, however, a genuine and recognisable symptom — slowed thinking, poor concentration, losing words mid-sentence, walking into a room and forgetting why.

It is also one of the more satisfying symptoms to investigate, because a large proportion of cases have a cause that is identifiable on a blood test and correctable.

Hormonal — perimenopause is far and away the most commonly missed cause in women over 40, and brain fog is frequently the symptom that bothers them most. Underactive thyroid slows cognition alongside everything else.

Deficiency — B12, iron and vitamin D. All common, all cheap to test.

Sleep — sleep apnoea is substantially underdiagnosed, particularly in women, and fragmented sleep produces exactly this picture. Insomnia likewise.

Mental health — depression, anxiety and burnout all impair concentration measurably.

Other — undiagnosed ADHD, particularly in women reaching their thirties and forties; post-viral fatigue including after COVID; medication side effects; alcohol; coeliac disease.

Twenty minutes matters here, because the history is the whole thing — when it started, what else changed at the same time, how you sleep, what you take, and in women over 40, where you are in your cycle.

Testing is broad because the causes are: our fatigue panel covers thyroid function, B12 and folate, ferritin, vitamin D, HbA1c and coeliac screening in one visit. Where perimenopause is plausible we add hormone testing, though in women over 45 the diagnosis is clinical.

We always ask about snoring and witnessed breathing pauses, because sleep apnoea is common, missed, and completely treatable.

Is brain fog a real thing, or am I just tired?

It is a real and recognisable symptom, even though it is not a diagnosis in itself. People describe it consistently: thinking through treacle, losing words mid-sentence, rereading the same paragraph, walking into rooms and forgetting why, finding ordinary work effortful in a way it never used to be.

It has causes, most of them identifiable and many of them treatable. It is not a character failing, and it is not something to be talked out of.

What tests are worth doing?

Thyroid function, B12 and folate, ferritin, vitamin D, HbA1c, full blood count, kidney and liver function, and coeliac screening.

That panel finds the reversible causes. Our tiredness panel covers most of it in one appointment. Note that "normal" B12 near the bottom of the range can still be symptomatic — the reference range is wide and worth interpreting rather than glancing at.

Is this early dementia?

In the overwhelming majority of people asking that question, no — and the distinction is reasonably clear.

Brain fog: you are acutely aware of it, it fluctuates from day to day, it is worse when you are tired or stressed, and the information is usually there if you slow down and reach for it. Insight is intact — the fact that you are worried is itself somewhat reassuring.

What warrants proper assessment: other people noticing before you do, getting lost in familiar places, repeating questions within a single conversation, struggling with long-familiar tasks, difficulty finding common everyday words, or personality change. That pattern deserves formal assessment, and there is a great deal to be gained from doing it — several causes of memory decline are treatable.

Could this be the perimenopause?

Very possibly, and it is one of the most under-recognised causes there is. Cognitive symptoms — fog, word-finding trouble, losing the thread — are common in perimenopause, can start in the early forties while periods are still regular, and are often the symptom women find most frightening because they read it as early dementia.

It is not. HRT frequently improves it substantially, and diagnosis over 45 is made on symptoms rather than a blood test, since hormone levels swing too widely to be informative.

Could it be ADHD?

It is worth considering, particularly if the difficulty is long-standing rather than new, and if it is about starting things, sustaining attention on the dull ones, and losing track rather than about being slowed down. Many adults — disproportionately women — are diagnosed late, having spent years describing it as brain fog or assuming they were simply disorganised. An assessment gives a proper answer either way.

Will exercise help, or make it worse?

It depends entirely on whether you have post-exertional malaise, and getting this wrong causes real harm.

For fog from poor sleep, low mood, stress or deconditioning, regular moderate exercise helps considerably. But for post-viral fog where exertion reliably leaves you worse a day or two later, pushing through makes things worse, and pacing within your limits is the correct approach. Work out which situation you are in before committing to a programme.

How long does it take to lift?

It depends on the cause, and honest ranges help. Treating B12 deficiency or an underactive thyroid: weeks to a few months. Starting HRT for perimenopausal fog: often noticeable within weeks. Treating sleep apnoea: sometimes within days, and the change can be dramatic. Post-viral fog: slower, measured in months, and improving unevenly rather than steadily.

What is not helpful is being told to live with it before those causes have been looked for.