If you made a list of the symptoms people describe as brain fog — slowed thinking, poor concentration, memory lapses, fatigue that sleep doesn't fix, low mood — you would have produced a fairly good description of an underactive thyroid.
It is common, it is found with a routine blood test, and it is treatable. Which makes it one of the highest-value things to exclude before concluding anything else — and one of the most commonly missed, because the cognitive symptoms usually arrive before anything that looks obviously medical.
What the thyroid does to the brain
Thyroid hormone sets metabolic rate in essentially every tissue, and the brain is metabolically expensive — it consumes a disproportionate share of the body's energy for its weight. Thyroid hormone receptors are present throughout it.
The mechanics are worth knowing because they explain the test. The thyroid mostly produces T4, which is relatively inactive. Tissues convert it locally to T3, the active form, using enzymes called deiodinases — and the brain does a substantial amount of its own local conversion. Meanwhile the pituitary monitors circulating thyroid hormone and adjusts TSH, the signal telling the thyroid to produce more.
Too little thyroid hormone and processing slows — subjectively, thinking through treacle. Too much and the system runs hot: fast, jittery, unable to settle.
Underactive: hypothyroidism
Slowed thinking and speech · poor concentration and memory · fatigue unrelieved by rest · low mood, sometimes indistinguishable from depression · cold intolerance · weight gain · dry skin and hair changes · constipation · heavier or irregular periods.
The cognitive and mood symptoms often arrive first and get attributed to stress, age or overwork. By the time the more recognisable physical signs appear, people have frequently spent a year or more assuming the problem was their workload.
In countries with adequate dietary iodine, the most common cause is Hashimoto's thyroiditis — an autoimmune process in which the immune system gradually damages the thyroid. It runs in families, is far more common in women, and often coexists with other autoimmune conditions. That is why a family history is a genuinely useful piece of information to bring to the appointment.
It can also cause a peripheral neuropathy — numbness, tingling or burning in the feet and hands — which is one of the more overlooked presentations. The other causes worth excluding are here, including one that is itself a supplement.
Overactive: hyperthyroidism
Anxiety and agitation · racing thoughts · insomnia · irritability · tremor · palpitations · heat intolerance and sweating · weight loss despite a normal or increased appetite · frequent bowel movements.
This one is frequently mistaken for an anxiety disorder and treated as one for a long time. The distinguishing feature is usually the physical picture running alongside — tremor, heat intolerance, palpitations, unexplained weight loss. If anxiety appeared suddenly in adulthood with that physical cluster attached, thyroid function is worth checking before concluding it is psychological. We've written about the line between anxiety as a feeling and anxiety as a disorder, and a medical cause is the first thing a decent assessment rules out.
One counter-intuitive presentation worth knowing: in older adults, hyperthyroidism can show up as apathy, withdrawal and low mood rather than agitation — essentially the opposite of the textbook picture, and easily read as depression or as dementia.
The test, and how to ask for it
Ask for TSH, and ideally free T4.
TSH is the standard first-line test, and there's a reason it's sensitive: because the pituitary amplifies small changes in thyroid hormone into larger changes in TSH, TSH often moves outside its range before T4 does. That makes it a good screen — but it also means a TSH result needs interpreting alongside symptoms rather than read as a verdict.
TPO antibodies are worth asking about if TSH is raised, since their presence points to Hashimoto's and to a higher likelihood of progressing to overt hypothyroidism.
Who is at higher risk: women, particularly after pregnancy and around menopause · anyone with a family history of thyroid or autoimmune disease · people with type 1 diabetes, coeliac disease or other autoimmune conditions · anyone who has had neck radiotherapy · and it becomes more common with age.
Two reasons a "normal" result can mislead — and one is our problem
One — high-dose biotin interferes with the test. This matters here more than anywhere else on this site, because biotin is in a great many hair, skin and nail supplements at doses far above dietary intake.
Many thyroid blood tests are immunoassays that use biotin as part of the detection chemistry. Circulating biotin from a supplement can interfere with that chemistry and shift results — classically producing a falsely low TSH alongside a falsely high T4, a pattern that looks like an overactive thyroid in someone whose thyroid is fine. Regulators have issued safety communications about exactly this, and cases of misdiagnosis have been reported.
The practical instruction: tell the person taking your blood every supplement you take, and ask whether to stop biotin beforehand. A short gap is usually enough, but the timing is a decision for whoever is ordering the test, not for us. We say this because a supplement company that stays quiet about its own category interfering with a thyroid test is not being useful to you.
Two — the reference range is a population statistic. Ranges are derived from what is typical across a population, so a result inside the range is not automatically right for you, and the interpretation differs in pregnancy and with age. Which leads directly to the next section.
The contested bit, stated honestly
Subclinical hypothyroidism — a mildly raised TSH with a normal free T4 — is genuinely debated. Whether to treat depends on how high the TSH is, whether symptoms are present, antibody status, age, cardiovascular risk and pregnancy plans. Trials of treating milder cases in older adults have been largely unconvincing on symptoms; the picture in younger people with antibodies and symptoms is different. Clinicians disagree, and anyone telling you there is an obvious answer is overselling it.
What we would say: if your result sits in that grey zone and you have symptoms, that is a conversation worth having properly — including a repeat test, since a single mildly raised TSH sometimes normalises on its own — rather than being told your bloods are normal and sent away.
If you are on levothyroxine, read this bit
This is directly relevant to anyone taking supplements, including ours.
Levothyroxine absorption is reduced by several things commonly taken at the same time of day — notably iron and calcium supplements, and magnesium- or aluminium-containing antacids. The standard advice is to take levothyroxine on an empty stomach and to separate it from these by around four hours. Coffee too close to the dose also reduces absorption for some people.
The consequence is quietly common: someone stable on levothyroxine starts a mineral supplement, takes it with their morning tablet, and their symptoms drift back. The dose didn't stop working; less of it is getting in.
If you take levothyroxine and any of our mineral products, separate them by several hours, and mention the supplement at your next thyroid blood test. That is a real interaction and we would rather you knew it than kept buying without knowing it. The wider set of magnesium interactions is here.
Iodine, and a warning
Iodine is genuinely required for thyroid hormone production, which is why iodine supplements are marketed for thyroid health.
Do not take high-dose iodine speculatively. In iodine-sufficient populations — which includes most of Western Europe and North America — excess iodine can cause thyroid dysfunction rather than fix it, and can worsen autoimmune thyroid disease. Kelp and seaweed supplements are a common and underestimated route to a large iodine dose. This is a test-first situation, not a supplement-first one.
Selenium has a more interesting literature in autoimmune thyroid disease, and it is also a nutrient with a comparatively narrow margin between adequate and too much. It is not something to self-prescribe at high doses either.
We don't sell iodine or kelp. The paragraphs above are what we'd write if we did.
Where this fits
Thyroid is the first of the four blood tests we'd suggest asking for if brain fog is persistent — the full list is here. If it comes back abnormal, you have found something treatable. If it is clear, you have excluded the most common reversible cause and the next questions get easier.
Two of those next questions: iron stores, where a normal full blood count can sit alongside low ferritin, and sleep — because if you sleep a full night and wake unrefreshed, no blood test will find the reason. And if the worry is specifically memory, this is the more useful article.
One further context worth naming: thyroid dysfunction is a standard part of the workup for widespread pain and fatigue, because it produces both — and chronic pain has its own set of guidelines worth knowing.
This article is not medical advice. Thyroid testing, interpretation and treatment decisions belong with a doctor, and nothing here should be used to adjust or stop prescribed medication.