We've written before about brain fog having causes rather than being a diagnosis. This is the practical follow-up: what to actually ask for at the appointment, how to prepare so the results mean something, and what to do with the numbers when they come back.
The reason to start here rather than with a supplement is simple arithmetic. Four of the most common causes of persistent cognitive fog are found by cheap, routine blood tests and are treatable once found. Working through the supplement aisle first can cost months on a problem a single test would have named.
The four tests
1. Thyroid function — TSH, and ideally free T4
An underactive thyroid produces slowed thinking, poor memory, fatigue and low mood — a near-perfect match for what people describe as brain fog. It's common, it's easily treated, and it's the first thing worth excluding.
TSH is the standard first-line test and is often sufficient on its own, because the pituitary amplifies small changes in thyroid hormone into larger changes in TSH. Free T4 adds useful information, particularly if TSH is borderline. If TSH comes back raised, TPO antibodies are the sensible follow-up, since they point to an autoimmune cause and to the likelihood of it progressing.
One thing to raise before the blood is taken: high-dose biotin can interfere with thyroid immunoassays and produce a falsely low TSH with a falsely high T4 — a pattern that mimics an overactive thyroid. Biotin is in a great many hair, skin and nail supplements. Say what you take and ask whether to pause it first. Full detail in thyroid and the brain.
2. Full blood count plus ferritin
Anaemia causes fatigue and poor concentration. But ferritin matters separately from haemoglobin, and this is the single most commonly missed thing on this list: you can have a completely normal full blood count and still have depleted iron stores.
Iron deficiency without anaemia is a recognised cause of fatigue and poor concentration, and it comes earlier in the sequence — stores fall first, and the blood count only changes once they're exhausted. So a "normal bloods" result that didn't include ferritin has not excluded iron deficiency.
Ask for ferritin explicitly, and ask for the actual number rather than accepting "normal" — many lab reference ranges start very low, and a figure at the bottom of a range is a different clinical situation from one in the middle. The threshold that matters varies by what you're investigating: it is notably higher in restless legs, where supplementation is indicated at levels most labs would report as normal.
One caveat worth knowing so you can interpret your own result: ferritin rises with inflammation and infection, so a recent illness can make a genuinely low store look adequate. If you were unwell around the time of the test, that's worth mentioning.
3. Vitamin B12 and folate
B12 deficiency causes cognitive symptoms and can do so before anaemia appears, which is why it gets missed when only a blood count is done.
Higher risk if you are vegetarian or vegan, over 60, take metformin or long-term acid-reducing medication (PPIs and H2 blockers reduce B12 absorption), have had gastric or bowel surgery, or have coeliac or Crohn's disease.
A detail worth knowing: serum B12 is an imperfect test, and results in the low-normal band are genuinely ambiguous. If your B12 sits low-normal and your symptoms fit, the standard next step is a functional marker — methylmalonic acid or homocysteine, both of which rise when B12 is functionally insufficient. That is the specific thing to ask about rather than accepting a borderline result as settled. Folate should be checked alongside, since the two interact. What the B12 trials say, and where they disagree.
One further reason not to leave a borderline B12 sitting: it can cause peripheral neuropathy, and that nerve damage becomes less reversible the longer it runs.
4. Vitamin D
The evidence linking vitamin D to cognition is more mixed than the supplement aisle suggests — deficiency is associated with poorer cognitive outcomes, but trials of supplementation in people who aren't deficient have been largely unimpressive. What makes it worth testing is that deficiency is genuinely common, particularly at northern latitudes and through winter, and correcting it is cheap and uncontroversial. The cognition and mood trials, in full.
And a fifth if it fits: HbA1c
Worth adding if you also have afternoon crashes, unusual thirst, increased urination, or a family history of diabetes. HbA1c reflects average blood glucose over roughly the previous three months, so unlike a single glucose reading it doesn't depend on what you ate that morning.
How to prepare, so the results are worth having
Small things that change whether a test tells you anything.
- List every supplement, including ours, and including anything you take occasionally. Biotin is the specific one for thyroid tests; high-dose B12 supplements will also make a serum B12 result uninterpretable.
- Ask whether to fast. Not needed for most of the four, but often requested if lipids or glucose are being added.
- Mention a recent illness — it raises ferritin and can mask deficiency.
- Bring your medication list, prescription and otherwise. Metformin and acid-reducers change the B12 interpretation; several other classes affect cognition directly.
- Ask for a copy of the numbers. "Your bloods are normal" is not a result. The numbers are, and you may want them later.
Two causes no blood test will find
Sleep apnea. If you sleep a full night and wake exhausted, this is the most likely single explanation and bloods won't show it. It needs a screener and a sleep study — start with the STOP-BANG questions. This one matters disproportionately because it is common, entirely invisible to the person who has it, and carries cardiovascular consequences if it goes untreated for years.
Depression. Cognitive symptoms — poor concentration, indecision, memory complaints — are part of the clinical picture, and often the part people notice first while assuming the low mood is a consequence of the fog rather than connected to it. It is worth raising directly, and it is treatable, and the cognitive symptoms improve when it is treated.
Two more that bloods won't catch: medication effects — sedating antihistamines, benzodiazepines and Z-drugs, some bladder medications, opioids, and the combined anticholinergic load across several of them — and simple chronic sleep restriction, which produces measurable cognitive impairment and is easy to underestimate in yourself.
And the situations that have their own explanation
If your fog has an obvious context, there is usually a more specific answer than a general blood panel:
- After a viral illness — long COVID brain fog has a describable mechanism, and it is not a nutrient problem.
- Around the menopausal transition — perimenopause brain fog is common, real, and frequently mistaken by the person experiencing it for something far worse.
- After having a baby — postpartum brain fog has its own research base, and sleep deprivation is only part of it.
- Alongside widespread pain and unrefreshing fatigue — chronic primary pain commonly includes cognitive symptoms, and the guidelines for it are counter-intuitive enough to be worth reading.
- In a man being sold a testosterone explanation — the symptom list overlaps almost exactly with untreated sleep apnea, and the order you investigate in matters.
None of those remove the case for the blood tests — iron deficiency after pregnancy is common, for instance — but they change what you're looking for.
How to make the appointment work
Go in with one sentence: "I've had persistent problems with concentration and memory for [how long]. I'd like to check thyroid, iron including ferritin, B12 and vitamin D."
Naming the tests is not being a difficult patient. It's giving a ten-minute appointment a starting point.
Take a note of: how long it's been going on · whether it's constant or fluctuating · what else changed around the time it started — a new medication, a life event, a change in sleep · whether anyone else has noticed · and every medication and supplement you take.
What to do with the results
Something is abnormal. Good — that's the useful outcome. Treat it, and give it time; thyroid and B12 correction in particular work over weeks to months, not days. Re-check symptoms once the numbers have normalised rather than assuming the fog will lift the same week.
Everything is normal but a result sits at the edge of its range. Worth a specific conversation rather than a shrug — especially a low-normal ferritin or B12, where the follow-up tests above exist for exactly this situation.
Everything is genuinely clear. Then the questions that remain are sleep (including apnea), mood, medication, alcohol, and load. All four are more likely explanations than a missing nutrient, and none of them is solved by buying something.
When it's more than a blood test question
Go back sooner, rather than waiting for results, if the fog is accompanied by: weakness, numbness or unsteadiness · new difficulty with speech or language · visual change · persistent headache · a change over weeks rather than months · or personality change others have noticed. If the specific worry is memory rather than concentration, this is the more useful article.
Where supplements fit
Narrowly, and after this.
If a test shows a deficiency, correcting it is worthwhile — that isn't a supplement claim, it's arithmetic. If the tests are clear and sleep apnea is excluded and mood and medication have been considered, then a cognitive supplement is a reasonable thing to try, with reasonable expectations.
Doing it in the other order means potentially spending months on capsules for a thyroid problem. There's a companion piece on when brain fog needs a doctor rather than a supplement.
This article is not medical advice. Which tests are appropriate for you, and what your results mean, are decisions for your doctor.