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Simon Parrott – Vitacuity Ltd
Iodine is a mineral your body cannot make but needs daily to produce thyroid hormones, which control how your brain uses energy and how neurons communicate. Without enough iodine, your thyroid cannot function properly, and your brain essentially runs in low-power mode. About one in three people with hypothyroidism (low thyroid hormone) show measurable cognitive decline, including slower thinking, memory problems, and difficulty concentrating. This brain fog is real and recognised by doctors, and it can improve when thyroid function is restored.
Mild iodine deficiency is surprisingly common even in developed countries like the UK, often going unnoticed as a cause of mental sluggishness. However, iodine is one of the few nutrients where both too little and too much cause problems. The evidence suggests that getting 150 micrograms daily through diet or a standard supplement is safe and practical for healthy adults without thyroid disease, but people with existing thyroid conditions should check with their doctor first.
If you experience brain fog alongside fatigue, feeling cold, or weight changes, a simple thyroid blood test can reveal whether inadequate iodine or thyroid function is the cause. The research shows the brain can recover once thyroid function is properly managed. However, treating a thyroid-stimulating hormone level that is simply in the upper-normal range as a problem, without actual hormone deficiency, is not currently supported by evidence.
Verdict: Iodine deficiency is a correctable cause of cognitive decline that deserves attention, but supplementation should focus on meeting adequate daily needs rather than chasing artificially optimal hormone levels.
What if the foggy thinking you’ve been blaming on stress, poor sleep, or simply “getting older” had a simpler, more correctable explanation? What if a tiny mineral, one your body cannot make itself and needs every single day, was quietly running low, and your brain was the first to feel it?
Iodine doesn’t get the attention of omega-3 or vitamin D in the wellness conversation. It’s rarely on the radar of people trying to protect their cognitive health in midlife. And yet, without it, your thyroid cannot produce its key hormones, and without those hormones, your brain simply doesn’t run properly. We’re not talking about exotic deficiency here. Mild iodine shortfall is common even in developed countries like the UK [1]. It’s the kind of gap that doesn’t show up as dramatic illness, it shows up as that nagging, hard-to-name mental sluggishness that so many people over 40 quietly accept as inevitable.
It isn’t inevitable. Here’s what the research actually says.
Vitacuity has analysed over 1.77 million research papers and selected the most relevant science on this topic. What follows is an honest, clear-eyed summary of what we know, what we don’t, and what a sensible, informed person might reasonably do about it.
To understand why iodine matters for cognition, you first need to understand what thyroid hormones actually do, and how completely dependent they are on iodine.
Your thyroid gland, a small butterfly-shaped structure at the base of your throat, produces two key hormones: thyroxine (T4) and triiodothyronine (T3). These hormones act as master regulators of cellular metabolism throughout the entire body, including the brain [10]. They govern how fast cells work, how efficiently they use energy, and how neurons grow, connect and communicate. Without adequate thyroid hormone, the brain essentially runs in low-power mode.
The catch? Your thyroid cannot manufacture these hormones without iodine. Iodine is the raw material. When you eat it, your body oxidises it and uses it to build T3 and T4 in a tightly regulated biochemical process [2]. If iodine is scarce, thyroid hormone production drops. The pituitary gland senses this and releases more thyroid-stimulating hormone (TSH), essentially urging the thyroid to work harder. If the shortfall continues, the thyroid can enlarge (goitre), hormone levels fall, and the body, including the brain, begins to slow down [10].
This is hypothyroidism at its core. And the cognitive consequences are far more significant than most people realise.
Evidence grade: Strong, systematic review and meta-analysis of 85 studies
A major 2025 systematic review and meta-analysis published in *Alzheimer’s & Dementia* pulled together findings from 85 studies to answer a direct question: how common is cognitive impairment in people with hypothyroidism, and how severe is it? [4]
The numbers are striking. Using the Mini-Mental State Examination (MMSE), the standard clinical tool for assessing cognitive function, the prevalence of cognitive impairment in hypothyroid patients was 0.29, meaning roughly one in three people with hypothyroidism showed measurable cognitive decline [4].
The association between TSH levels (the marker that rises when thyroid hormone is low) and MMSE scores was a correlation of r = -0.46, a moderately strong negative relationship, meaning the higher the TSH, the lower the cognitive test score [4].
On memory specifically, hypothyroid patients scored significantly worse than healthy controls on both the MMSE (standardised mean difference of -1.134) and the Wechsler Memory Scale (SMD = -1.286) [4]. They also showed longer P300 latency (SMD = 1.124), a measure of brain processing speed captured by electrical recordings. Slower P300 means the brain is taking longer to respond to information.
Neuroimaging data from these studies also revealed structural and functional changes in the brains of hypothyroid patients who had not yet started treatment [4]. This isn’t just a functional issue, at a measurable, physical level, low thyroid hormone appears to alter how the brain is organised and how it metabolises energy.
Evidence grade: Promising, clinical review with supporting studies
“Brain fog” is a term that gets dismissed too often in clinical settings. But a 2022 paper published in *Thyroid* tackled it directly, asking: what is brain fog in hypothyroidism, how do we measure it, and what can actually be done? [13]
What makes this paper significant is its frank acknowledgement that cognitive symptoms in hypothyroidism are not just subjective complaints, they are measurable, they are real, and they affect quality of life in ways that standard thyroid blood tests don’t always capture. Patients describe difficulty concentrating, slowed thinking, word-finding problems, and memory gaps that feel qualitatively different from normal fatigue [13, 14].
A 2022 study in *Journal of Neuroscience Nursing* added a practical dimension: in 441 people with hypothyroidism who were prescribed thyroid hormone replacement, those who were non-adherent to their medication reported significantly greater brain fog, including impairments in prospective memory (remembering to do things), retrospective memory (recalling past events), and short- and long-term recall [15]. The cruel irony is that the brain fog itself may make it harder to remember to take the medication that treats it.
A 2024 paper in *BMJ* framed the question starkly in its title: *Brain fog in hypothyroidism: recovery or dementia?* [14] The answer appears to be that recovery is possible, but it’s not guaranteed, and the cognitive changes associated with untreated or undertreated hypothyroidism can be significant enough to be mistaken for early dementia. Getting the thyroid right matters enormously.
Evidence grade: Promising, small human fMRI study with pre/post treatment comparison
Here’s where things get particularly relevant for people in midlife who feel cognitively “off” but whose blood tests are told are “borderline” or “subclinical.”
A 2006 study published in *Neuroimage* used functional MRI (fMRI) to look directly at brain activity in three groups: people with normal thyroid function, those with subclinical hypothyroidism (SCH, meaning TSH is elevated but T4 is still in range), and those with hyperthyroidism [9].
All participants performed a digit n-back working memory task, a standard cognitive test, while their brains were scanned. The SCH patients scored significantly lower on the 2-back task compared to those with normal thyroid function (p < 0.012). More importantly, the fMRI revealed *why*: in SCH patients, the normal pattern of activity in the frontoparietal network, including the dorsolateral prefrontal cortex, middle and inferior frontal gyri, and supplementary motor area, was disrupted [9]. These are the brain regions central to working memory, planning, and executive function.
Then the SCH patients were treated with L-thyroxine (thyroid hormone replacement) for approximately six months. When re-scanned, their brain activation patterns had normalised, matching those of the healthy control group [9]. Their performance on the working memory task also improved.
This is an important finding: it suggests that the cognitive effects of subclinical hypothyroidism are not permanent. Address the underlying thyroid problem, and the brain appears to recover its function. The sample was small, so we shouldn’t overgeneralise, but the biological signal here is clear and meaningful.
Evidence grade: Strong, multiple reviews and observational data
Many people assume iodine deficiency is a problem confined to low-income countries without access to iodised salt. The evidence suggests otherwise.
Multiple 2025 reviews confirm that mild iodine deficiency is common even in developed countries [1, 2]. In the UK specifically, iodine intake has been a documented public health concern, particularly since the shift away from iodised salt and the rise of plant-based diets (which tend to exclude dairy and fish, two of the main dietary iodine sources in Western diets).
A 2022 review in *Critical Reviews in Food Science and Nutrition* notes that iodine deficiency is one of the most common causes of thyroid disorders worldwide, and that while severe deficiency (causing goitre and cretinism) has been largely addressed in many countries through iodised salt programmes, *mild* deficiency remains widespread [10]. This mild shortfall is enough to cause thyroid overstimulation, altered hormone ratios, and, critically, the kind of low-grade cognitive underperformance that people often attribute to other causes.
Iodine requirements are not static throughout life either. Pregnancy dramatically increases them [1, 8]. But for adults in midlife, the 40 to 65 age group, the daily requirement is 150 micrograms, and many people are not consistently meeting it through diet alone.
Evidence grade: Promising for excess effects, animal data, supported by human observational evidence
Iodine occupies a relatively narrow optimal range. This is worth understanding clearly, because “more is better” logic does not apply here.
Excessive iodine intake can paradoxically suppress thyroid hormone production through a mechanism called the Wolff-Chaikoff effect, where the thyroid temporarily downregulates iodine uptake when exposed to a sudden surge [1]. In the short term, this is a protective mechanism. But in vulnerable individuals, particularly those with pre-existing thyroid conditions or autoimmune thyroiditis, prolonged high iodine intake can lead to hypothyroidism [10].
A 2025 animal study examined what happens when mice are exposed to excessive iodine during the perinatal period, finding effects on thyroid function and higher brain functions including learning and memory [3]. While this is animal data that cannot be directly extrapolated to adult humans, it reinforces the biological principle that iodine excess is not harmless.
The public health picture tells a similar story. Countries that introduced iodised salt programmes saw the near-elimination of severe goitre and cretinism, but also observed a subsequent rise in autoimmune thyroid conditions like Hashimoto’s thyroiditis in some regions [10]. Too little and too much are both problematic. The goal is adequacy, not maximisation.
Evidence grade: Conflicted, see explanation below
One of the more contentious areas in thyroid and brain health research is whether people whose TSH sits at the higher end of the “normal” range, but hasn’t crossed the clinical threshold for hypothyroidism, are at meaningful cognitive risk.
Some clinicians and functional medicine practitioners argue that TSH above 2.0 μIU/mL (even within the conventional normal range of up to 4.5 μIU/mL) represents “suboptimal” thyroid function that may impair cognition. A 2025 study in *Alzheimer’s & Dementia*, drawing on data from 688 participants in the Canadian Comprehensive Assessment of Neurodegeneration and Dementia cohort (ages 50–91), tested this hypothesis directly [5].
The finding? No significant association was found between higher-normal TSH levels or lower-normal free T4 and cognitive impairment [5]. Critically, among participants with elevated TSH, 98.6% had normal free T4 levels, suggesting that elevated TSH in itself, without accompanying hormone deficiency, may not be clinically meaningful for cognition. The authors concluded that redefining the TSH cutoff to 2.0 μIU/mL lacks clinical justification, and they cautioned against prescribing thyroid supplementation for cognitive health in people with normal thyroid function [5].
Why does this conflict with other findings? The key distinction is between *clinical hypothyroidism* (where thyroid hormone is actually low) and *higher-normal TSH within a functioning system*. The cognitive impairment documented in the meta-analysis [4] and the fMRI study [9] relates to genuine thyroid hormone insufficiency, not simply a TSH reading at the top of normal. The overall weight of evidence suggests that iodine adequacy and thyroid health matter enormously for cognition, but that treating a normal TSH as a problem requiring intervention is not currently supported by the data.
The nuanced truth: make sure your thyroid is working properly. If it isn’t, address it. But don’t medicalise a TSH reading that sits within normal range.
Honest uncertainty is part of good science. Here’s what the current research leaves open:
The brain fog question is still partially unresolved. While we know brain fog is real and measurable in clinical hypothyroidism [13], and that treatment can help [9], some patients continue to report cognitive symptoms even after their TSH and T4 return to normal ranges. Why this happens, whether it reflects lingering neurological changes, the T3/T4 conversion issue, or other factors, is not yet fully understood [14].
The mild deficiency threshold for cognition in adults is unclear. Most research on iodine and cognitive function focuses on severe deficiency or clinical hypothyroidism. The cognitive impact of mild-to-moderate iodine deficiency in otherwise healthy adults in midlife, people who are not clinically hypothyroid but may be running below optimal intake, has not been rigorously studied with randomised controlled trials [8].
The “suboptimal TSH” debate remains active. The 2025 Canadian cohort study [5] found no cognitive link with higher-normal TSH, but this is a single cohort. Longer-term, larger studies are needed to settle this question definitively.
Almost all perinatal and developmental iodine research focuses on pregnancy and infancy. The effects of iodine status on adult and ageing brains specifically, a key gap for this audience, have been studied mainly through the lens of thyroid disease, not iodine nutrition directly [8]. We are drawing reasonable inferences from the thyroid-cognition relationship, but the direct iodine-supplementation-cognition RCTs in healthy adults simply don’t exist yet in sufficient numbers.
The excess iodine and brain function data comes largely from animal models [3]. How this translates to adult human supplementation at typical doses is not yet established.
Here’s what a sensible, informed person should take away from all of this.
First, recognise that iodine matters for your brain, via your thyroid. The chain of causation is well-established and strong: iodine deficiency → reduced thyroid hormone → impaired cognitive function [2, 4, 10]. Roughly one in three people with hypothyroidism has measurable cognitive impairment [4]. Brain fog in this context is real, not imagined [13]. And the effects appear to be reversible when thyroid function is restored [9].
Second, mild iodine deficiency is more common than you probably think in the UK. Even in developed countries, mild shortfall is documented and often goes unnoticed [1, 10]. It doesn’t announce itself with dramatic symptoms. It just makes thinking feel harder.
Third, the good news about iodine as a supplement, it’s water-soluble. Iodine is excreted in urine, which means excess at normal supplement doses is typically cleared by the body. Supplementing at the recommended daily amount (150 micrograms for adults) is safe, practical, and unlikely to cause harm in people without pre-existing thyroid disease. The recommended upper intake level is around 600 micrograms per day for adults, so there’s a sensible margin. Supplement daily at a standard dose, don’t megadose.
However, the Goldilocks caveat is real. Iodine is one of the few micronutrients where both deficiency and excess are meaningfully problematic [10]. If you have a known thyroid condition, particularly Hashimoto’s thyroiditis, check with your doctor before supplementing, as high iodine intake can worsen autoimmune thyroid conditions in some people. For healthy adults without thyroid disease, a standard iodine supplement or a diet reliably including dairy, fish, or eggs most days is a sensible, low-cost strategy.
Fourth, if you have genuine brain fog, particularly if it comes with fatigue, feeling cold, weight changes, or hair thinning, get a thyroid blood test. TSH is inexpensive and widely available. If your TSH is elevated and your free T4 is low, this is a clinical matter that deserves medical attention and proper treatment. The fMRI data shows that the brain can recover once thyroid function is properly managed [9]. Don’t just accept cognitive decline as inevitable without ruling out something this correctable.
Fifth, don’t chase an artificially “optimal” TSH below 2.0. The evidence does not currently support treating a TSH in the upper-normal range as a cognitive problem requiring thyroid hormone supplementation [5]. That’s a different conversation from ensuring adequate iodine nutrition, and conflating the two leads to unnecessary intervention.
The practical upshot: ensure you’re getting at least 150 micrograms of iodine daily through diet or supplementation, take a standard supplement if your diet is dairy-free, fish-free, or plant-based, and get your thyroid checked if you have symptoms that fit. Your brain runs on thyroid hormones. Thyroid hormones run on iodine. The supply chain matters.
[1] Critical Role of Iodine and Thyroid Hormones During Pregnancy (2025). DOI: 10.3390/ijms262110247 | https://pubmed.ncbi.nlm.nih.gov/41226288/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12610087/
[2] Iodine and Hypothyroidism (2025). DOI: 10.2174/0118715303355789250321080037 | https://pubmed.ncbi.nlm.nih.gov/40621769/
[3] Effects of excessive iodine intake during the perinatal period on thyroid function and higher brain functions in mouse offspring (2025). DOI: 10.1507/endocrj.EJ24-0723 | https://pubmed.ncbi.nlm.nih.gov/40436777/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12436078/
[4] Prevalence, hormonal correlates, severity, and neural basis of neurocognitive impairment in patients with hypothyroidism: Systematic review and meta-analyses (2025). DOI: 10.1002/alz.70924 | https://pubmed.ncbi.nlm.nih.gov/41298253/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12657124/
[5] Thyroid function and cognitive health: rethinking the relationship between “suboptimal” TSH/FT4 levels and cognitive decline (2025). DOI: 10.1002/alz.70960 | https://pubmed.ncbi.nlm.nih.gov/41400007/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12706624/
[8] Optimizing Growth: The Case for Iodine (2023). DOI: 10.3390/nu15040814 | https://pubmed.ncbi.nlm.nih.gov/36839172/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9959690/
[9] fMRI revealed neural substrate for reversible working memory dysfunction in subclinical hypothyroidism (2006). https://pubmed.ncbi.nlm.nih.gov/16921178/
[10] The impact of the micronutrient iodine in health and diseases (2022). DOI: 10.1080/10408398.2020.1843398 | https://pubmed.ncbi.nlm.nih.gov/33226264/
[13] Brain Fog in Hypothyroidism: What Is It, How Is It Measured, and What Can Be Done About It (2022). DOI: 10.1089/thy.2022.0139 | https://pubmed.ncbi.nlm.nih.gov/35414261/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9469742/
[14] Brain fog in hypothyroidism: recovery or dementia? (2024). DOI: 10.1136/bmj.q1426 | https://pubmed.ncbi.nlm.nih.gov/38960619/
[15] The Relationship Between Brain Fog and Medication Adherence for Individuals With Hypothyroidism (2022). DOI: 10.1177/10547738211038127 | https://pubmed.ncbi.nlm.nih.gov/34348493/
This article is for informational purposes only and does not constitute medical advice. Food supplements should not be used as a substitute for a varied and balanced diet and healthy lifestyle. If you are pregnant, breastfeeding, taking medication or have a medical condition, consult your doctor before taking any supplement. These statements have not been evaluated by the Food and Drug Administration (FDA) or the Medicines and Healthcare products Regulatory Agency (MHRA). This product is not intended to diagnose, treat, cure, or prevent any disease.
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