Hypothyroidism is a hormone imbalance with strong effects in most organs of the body. In the case of the brain, research suggests it causes brain fog with cognitive problems, mood changes, and fatigue. Read on to learn more about its causes, symptoms, and how to fix it.

How Hypothyroidism Affects the Brain

The Hypothalamus-Pituitary-Thyroid (HPT) Axis

The thyroid is a gland located at the base of the neck, below Adam’s apple. It produces the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which control the growth, development, and metabolism of nearly every cell in the body [1+].

The production of these hormones is regulated by a system involving the hypothalamus, pituitary, and thyroid gland – the hypothalamus-pituitary-thyroid axis.

The hypothalamus produces thyrotropin-releasing hormone (TRH), which activates the pituitary to produce and release thyroid-stimulating hormone (TSH). TSH, in turn, stimulates T3 and T4 production in the thyroid gland [1+, 2+].

The thyroid gland produces T4 from iodine and the amino acid tyrosine. T4 is then broken down to its more active form T3 by a group of enzymes called deiodinases and mainly found in the liver and kidneys [1+, 3+].

High levels of thyroid hormones block TRH and TSH production. This mechanism fine-tunes T3 and T4 production and maintains their levels within normal values [2+].

What Is Hypothyroidism?

Hypothyroidism is a deficiency in thyroid hormone production by the thyroid gland. Its main symptoms include [4+, 5+]:

  • Fatigue and weakness
  • Reduced memory and concentration
  • Depression
  • Cold intolerance
  • Constipation
  • Weight gain
  • Dry skin and hair loss
  • Painful joints and muscles

Hypothyroidism can be caused by defects in the thyroid gland (primary hypothyroidism) or the hypothalamus or pituitary gland (secondary hypothyroidism). These defects can be due to [4+, 5+]:

  • Iodine deficiency
  • Inborn abnormalities
  • Autoimmune damage
  • Injuries or surgical removal
  • Medication and radiotherapy
  • Inflammation or oxidative stress

Additionally, hypothyroidism can be classified based on the blood levels of the main hormones [6+, 5+]:

  • Overt hypothyroidism: low thyroid hormones and high TSH
  • Subclinical hypothyroidism: normal thyroid hormones and high TSH
  • Sick euthyroid syndrome: low thyroid hormones and normal to low TSH
  • Resistance to thyroid hormone: high thyroid hormones and TSH

How Does It Affect the Brain?

The main effect of low thyroid hormone and high TSH levels on the brain is a size reduction in the region that controls memory and learning (hippocampus). Hypothyroidism prevents the birth and branching of brain cells, and their ability to form and strengthen connections [7, 8, 9, 10, 11].

Low thyroid hormones also reduce oxygen supply (through a reduced blood flow) and sugar breakdown in regions controlling memory and attention (hippocampus, amygdala, and anterior cingulate cortex). This may result in a reduced production and usage of the energy molecule ATP [12, 13+, 14, 15].

Hypothyroidism also reduces brain signal transmission. It blocks a key enzyme to this process (Na-K-ATPase) and activates acetylcholinesterase, which breaks down the neurotransmitter acetylcholine. This would lead to lower acetylcholine levels. In a small trial on 19 people with hypothyroidism, over 50% had delayed brain signal transmission [16, 17].

Low thyroid hormone levels also increase protein levels in spinal fluid, suggesting they damage the blood-brain barrier [18].

Alternatively, subclinical hypothyroidism reduced brain activity in regions associated with verbal and spatial working memory (frontal gyri, prefrontal cortex, cingulate cortex, and parietal lobe) in 2 studies [19, 20].

Brain Fog from Hypothyroidism

What Is Brain Fog?

Brain fog, also known as ‘mental fog’, ‘clouding of consciousness’, and ‘cognitive dysfunction’, describes a constellation of cognitive symptoms such as [21, 22]:

  • Reduced mental clarity and cognitive function
  • Difficulty focusing and multitasking
  • Loss of short- and long-term memory
  • Slow thinking
  • Confusion
  • Fatigue

Because these symptoms are generally subjective, doctors can consider them too mild or unspecific to diagnose cognitive impairment.

Brain fog can have many causes, but they all involve inflammation and free radicals damaging the brain region responsible for emotions, cognitive, and executive function – the limbic system [23, 21, 24+].

Factors and conditions that may trigger brain fog include [22, 21, 25, 26, 27, 28]:

  • Anxiety and stress
  • Depression
  • Sleep problems
  • Hormone imbalances
  • Infections
  • Toxins
  • Diet
  • Drugs and medication
  • Medical conditions such as multiple sclerosis, lupus, and fibromyalgia

People with undiagnosed or poorly managed hypothyroidism often report forgetfulness, difficulties finding the right words, and lack of attention. But does this mean that hypothyroidism is a brain fog trigger? Let’s look at the research to find out.

Overt Hypothyroidism

This condition is clearly linked to brain fog symptoms, especially in elderly people, as seen in 6 observational studies on over 200 people. The main one was reduced memory (especially verbal) but some studies also reported problems with [29, 30, 31, 32, 33, 34]:

  • General intelligence
  • Attention
  • Learning capacity
  • Visual-spatial abilities
  • Coordination

The symptoms are generally reversible with thyroid replacement therapy, even in the most severe cases, as seen in 3 clinical trials on 56 adults and 58 children [35, 36, 37].

However, cognitive symptoms may persist in a few people. For instance, a study on over 100 people found reduced memory and attention despite following this therapy for 5.5 years [38, 39].

Untreated hypothyroidism may lead to dementia in adults and irreversible brain damage in children with inborn hypothyroidism [40+, 41+].

Subclinical Hypothyroidism

The case of subclinical hypothyroidism is more controversial. It was associated with mild cognitive problems in 10 studies on over 4k people but not in 9 other studies on over 11k. The most common symptom was reduced memory, with some studies reporting slower thinking, poor attention, and fatigue [42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60].

Meta-analyses found insufficient or only weak evidence to associate subclinical hypothyroidism with cognitive problems. Generally, the most severe cases (those with the highest TSH levels) were most likely to experience symptoms [61, 62, 63, 64].

Subclinical hypothyroidism rarely requires therapy. In the most severe cases, the symptoms can be reversed with thyroid hormone replacement [65+, 44, 19, 20].

Hashimoto’s Encephalopathy

Hashimoto’s thyroiditis is an autoimmune disease in which the immune system produces antibodies that target and progressively damage the thyroid gland. This may cause hypothyroidism and possibly brain fog [66+].

Alternatively, there is a rare type of autoimmune brain disease that may arise in stroke-like episodes or progressively damage the brain. Although its link to Hashimoto’s thyroiditis remains unclear, the condition is called Hashimoto’s encephalopathy because it’s also associated with high levels of antibodies against the thyroid gland [67+, 68+, 69+].

In addition to seizures, psychosis, and behavioral changes, Hashimoto’s encephalopathy may cause progressive brain fog symptoms such as [70+, 71, 72, 73, 74, 75, 76, 77]:

  • Slow thinking
  • Difficulties concentrating
  • Impaired executive function
  • Memory loss
  • Disorientation and confusion
  • Speech disorders

Brain imaging studies of people with this condition showed varied results. Some brains had a normal appearance, while others had alterations in white matter, reduced blood supply, swelling, and injuries [78, 79, 80+, 81+, 82+].

Scientists believe that thyroid antibodies may damage the brain (including myelin) or cause inflammation in the brain blood vessels [78, 83+].

Euthyroid Sick Disease

Euthyroid sick disease, also known as non-thyroidal illness, is a condition in which people with a functional thyroid gland have low thyroid hormone levels. This situation is most common in people who are starved, severely ill, or have undergone major surgery [84+].

Major surgery triggers the production of the cytokine IL-6 to stimulate the stress response. High levels of this cytokine are associated with lower T3 levels and reduced conversion of T4 into T3. Sedatives further reduce T3 levels by blocking its production in the liver [85, 86, 87, 88].

The resulting hypothyroidism, together with the toxicity of sedatives to the brain and frequent fasting, may cause brain fog in people undergoing major surgery [89, 90, 84+].

Euthyroid sick disease may also occur in people with less severe conditions.

In a study on 9 women, exposure to indoor air microbes caused reversible euthyroid sick disease with fatigue and cognitive problems [91].

In another study on over 600 people, euthyroid sick disease was more frequent among the very elderly (over 100 years old). In this group, it was associated with worse cognitive performance and health status [92].

Resistance to Thyroid Hormone

Resistance to thyroid hormone is a condition in which TSH levels remain high despite having normal to high thyroid hormone levels. The condition is due to mutations in the thyroid hormone receptors [93+, 94].

It doesn’t cause brain fog, but people with this condition often have ADHD or an IQ below average. Thyroid replacement therapy may reverse the symptoms [95, 96, 97, 98, 99, 100, 101, 102].

Effect of Thyroid Medications on the Brain

Levothyroxine Alone

Levothyroxine (LT4) is a synthetic form of T4. It’s routinely used as replacement therapy for hypothyroidism, with proven evidence of its effectiveness [4+].

LT4 reduced memory deficits in a clinical trial on 24 elderly people with hypothyroidism and maintained its effectiveness in the long term in a study on over 1k. Similarly, it improved cognitive function and behavior at school in a clinical trial on 18 children [36, 103, 37].

The effectiveness of this synthetic hormone in people with subclinical hypothyroidism is less clear. LT4 improved memory in 4 trials on 135 adults and 20 children, but not in 3 trials on over 200 adults [31, 104, 105, 106, 107, 108, 109].

The current management guidelines for subclinical hypothyroidism recommend using LT4 preferably in younger people and those with more severe conditions (higher TSH levels) [110].

Combined Therapies

Some people complain about hypothyroidism symptoms even after LT4 therapy. Despite the popular belief that adding T3 may increase its effectiveness, the combination wasn’t better than LT4 alone in 3 clinical trials on almost 300 people [111, 112, 113].

Based on this, 4 meta-analyses recommended maintaining LT4 alone as standard therapy for hypothyroidism [114, 115, 116, 117].

However, the combination may be preferred in people with reduced T3 production. This includes those with mutated variants of the enzyme that breaks down T4 into T3 (type 2 deiodinase), thyroid cancer, or who have undergone thyroid removal [118+, 119, 120, 121].

Desiccated thyroid extract is a mix containing both T3 and T4. In a clinical trial on 70 people with hypothyroidism, this extract was as effective as LT4 alone at improving the symptoms (including cognitive problems). An advantage was that people on this therapy tended to be more satisfied, possibly because it made them lose weight [122, 123].

Adding vitamin E to LT4 therapy may increase its effectiveness for cognitive symptoms because it protects the brain from free radicals. This combination was better than LT4 alone in rats [124].

Health Risks of Brain Fog from Hypothyroidism


A study on 70 people with a family history of Alzheimer’s disease found a high prevalence of autoimmune hypothyroidism (41%), suggesting that the genes responsible for both conditions are inherited together [125].

The association of subclinical hypothyroidism with Alzheimer’s is weak. A study on almost 2k elderly people found it increased the risk only in women, while 3 other studies on almost 2k found no link between both conditions [126, 127, 128, 129].

Similarly, subclinical hypothyroidism – as opposed to hyperthyroidism – wasn’t associated with an increased risk of dementia in 2 studies on over 12k people. [130, 131].

In turn, another study on over 650 people associated this condition with an increased risk of brain damage due to reduced blood supply (vascular dementia) [129].

The effects of a history of thyroid replacement therapy are also unclear. A study on ~500 people associated it with an increased risk of Alzheimer’s, while another one on over 2500 didn’t [132, 133].

Heart and Blood Vessel Disease

Subclinical hypothyroidism was associated with an increased risk of heart disease (such as heart attack, congestive heart failure, and irregular heart rate) in 4 studies on over 9k people [134, 135, 136, 137].

Similarly, high blood cholesterol, clogged arteries, and brain strokes were more frequent among people with a history of thyroid replacement therapy in a study on over 27k people [133].

Natural Ways to Help Brain Fog from Hypothyroidism

Lifestyle Changes

1) Do More Exercise

Doing more exercise may improve how your thyroid gland works. In 2 small trials on 20 adults and 36 adolescents, exercise increased T3 and T4 while decreasing TSH levels [138+, 139+].

What’s more, exercise is beneficial for cognitive function, especially if it combines aerobic and strength training. In 4 small trials on almost 100 people with cognitive problems, it improved learning, processing speed, attention, executive function, and movement coordination [140+, 141, 142, 143, 144].

Exercise may increase cognitive function by stimulating the birth of new cells, increasing synaptic plasticity, reducing inflammation and oxidative damage, and improving blood flow in the brain [145+, 146+, 147, 148+, 140+].

However, extenuating exercise may do more harm than good. It reduces T3 levels, increases inflammation (Th17 immune response), and causes fatigue, a well-known brain fog trigger [149, 150, 151, 152].

Experts recommend moderate exercise such as walking, yoga, tai chi, or water aerobics.

2) Improve your Sleep

Poor sleep quality is a common cause of brain fog. Lack of sleep reduced attention, memory, creativity, language and numerical skills, and executive function in multiple studies [153, 154, 155, 156, 157, 158].

Sleep loss increases free radical damage in the hypothalamus by reducing the production of the antioxidant glutathione. It also increases the production of pro-inflammatory cytokines (such as TNF-alpha, IL-6, IL-17, and CRP) [159, 160, 161, 162, 163, 164].

If you have brain fog from hypothyroidism, you should try to improve your sleep quality. Read this post to learn how to fix your sleep.

People with interrupted breathing during sleep (sleep apnea) are at an increased risk of brain fog. In addition to worsening sleep quality, apnea reduces oxygen intake. The lack of oxygen produces free radicals, which cause oxidative damage and trigger inflammation [165+].

Importantly, sleep apnea increases the risk of developing Hashimoto’s hypothyroidism. The risk is especially high in women and those with severe apnea. Similarly, apnea increases thyroid antibody levels in people with Hashimoto’s, suggesting the development of the symptoms may be faster [166].

3) Reduce Stress

Stress decreases thyroid function. Repeated stress increases glucocorticoid production, which in turn reduces the levels of T3, T4, and TSH [167, 168+].

Both chronic stress and the hormones produced in these conditions impair memory and learning. While the effects are reversible in adults, they may be permanent in young children [169, 170].

Stress reduces the birth and branching of brain cells in the hippocampus and the function of the messenger glutamate in the prefrontal cortex [169, 171, 172].

Alternatively, the stress hormone CRH may contribute to brain fog by increasing inflammation. CRH increases Th1 dominance and activates the inflammatory hub NF-kB. However, it also leads to the production of the anti-inflammatory hormone cortisol [173, 174, 175, 176+].

Read here how to reduce your stress response.


4) Check if You Have Gluten Intolerance

In addition to its well-known effects on the gut, celiac disease damages the brain by causing inflammation and producing antibodies that target its proteins [177+].

People with celiac disease often experience brain fog with reduced memory, attention, executive function, and processing speed after eating. In a small trial on 11 people recently diagnosed with this condition, going gluten-free improved brain fog symptoms [25, 178].

The frequency of celiac disease is higher in people with hypothyroidism, especially in those with autoimmune types such as Hashimoto’s [179, 180, 181].

Checking if you have gluten intolerance is highly recommended if you have brain fog from hypothyroidism.

5) Prevent Insulin Resistance

Diets rich in low-glycemic sugars and fats (a combination typically found in fast food) cause obesity and insulin resistance. This means that the body’s cells don’t take up sugar from the blood in response to insulin [182+, 183+].

Insulin resistance causes acute fluctuations in blood sugar levels, which increases oxidative damage and inflammation. This damages the brain and contributes to brain fog [184+, 185].

Obesity triggers insulin production in the hypothalamus at lower blood sugar concentrations, resulting in higher insulin levels [186].

High blood sugar and insulin lower NAD+ levels. This reduces the ability of the mitochondria to produce the energy molecule ATP, resulting in increased fatigue [187, 188].

Reducing dietary carbohydrates may be especially beneficial in people with autoimmune hypothyroidism. In a clinical trial on almost 200 people with Hashimoto’s, a low-carbohydrate diet reduced thyroid antibody production [189].

Read here what causes insulin resistance and how to fix it.

6) Reduce Goitrogens

Goitrogens are substances that reduce iodine uptake by the thyroid gland, resulting in a reduced production of thyroid hormones [190+, 191+].

Foods rich in goitrogens include [192+]:

  • Cruciferous vegetables (cabbage, broccoli, cauliflower)
  • Root vegetables (cassava, turnip)
  • Cereals (sorghum, millet, maize)
  • Legumes (soybean, Lima bean)

Reducing your intake of these foods may improve hypothyroidism and decrease your risk of brain fog.

7) Protect Your Gut

Reduce Inflammation

Gut inflammation produces cytokines that may ultimately cause cognitive problems and behavioral changes. Although whether mental fatigue is caused by gut inflammation remains unclear, people with chronic fatigue syndrome often have IBS as well [193, 194, 195].

Some foods can increase gut inflammation and worsen IBS symptoms, possibly increasing the risk of brain fog. These include those rich in [196, 197+, 198, 199+, 200, 201+, 202]:

  • Lectins (such as beans, peanuts, lentils, tomatoes, and eggplants)
  • Casein (dairy products)
  • FODMAPs (such as many fruits, vegetables, cereals, and dairies)
  • Salicylates (such as apricots, oranges, pineapples, dates, and raspberries)
  • Amines (such as chocolate, cheese, wine, beer, and fish)
  • Tannins (such as bananas, chocolate, tea, nuts, and whole spices)
  • Trypsin inhibitors (such as legumes, cereals, potatoes, and eggs)
  • Oxalates (such as leafy vegetables, dark chocolate, legumes, and nuts)
  • Yeast (such as gluten-free bread)
  • Food additives like carrageenan and carboxymethylcellulose (such as ice cream, almond milk, candy bars, and some cheeses)
  • Caffeine (such as coffee, tea, and cocoa)

To find out if any of these food groups are worsening your brain fog from hypothyroidism, you can follow an elementary diet for 2 weeks and then add one at a time back in.

Preserve Your Microbiome

The gut microbiota plays a key role in cognitive function and behavior. The communication between these two organs is often called the ‘brain-gut axis’ [203].

In animal studies, a Western diet rich in saturated fats and added sugars reduced beneficial bacteria in the gut. This change is associated with obesity and cognitive impairment [204+, 205, 206].

Probiotics improved cognitive function and the gut microbiome composition in clinical trials. They improved visual memory in 22 healthy people, attention and memory in 50 people with mild cognitive impairment, and visual learning, attention, and general fatigue in 44 people with chronic fatigue syndrome [207, 208, 209].

They also improved learning and memory in mice and rats. Conversely, changes in the gut microbiota (from infections and antibiotics) caused cognitive problems and behavioral changes in mice [210, 211, 212, 213, 214, 215].

Interestingly, hypothyroidism is associated with bacterial overgrowth in the small bowel. This means it may worsen brain fog by disturbing the microbiome [216].


The following supplements can help you restore your thyroid hormone levels:

In the case of vitamin D, it’s important to combine dietary sources and supplements with sun exposure [230+, 231].

Additionally, vitamin B1 increases energy by stimulating carbohydrate breakdown. Supplementation with his vitamin reduced fatigue in 3 people with Hashimoto’s hypothyroidism [232, 233].

Nevertheless, it’s preferable to take certain vitamin and mineral supplements only in cases of deficiency. Some of them may be toxic at excessive doses and even worsen hypothyroidism or brain fog.

This is especially the case of iodine. Although it’s required to make thyroid hormones, high doses may trigger inflammation by increasing T cells (Th1 and Th17) and cytokines (such as IL-6, IL-17, and TGF-beta). This may worsen both brain fog and autoimmune hypothyroidism [234+, 235].

Limitations and Caveats

Most studies investigating an association between hypothyroidism and brain fog symptoms were cohort studies. These studies can’t establish a cause-effect relationship.

Additional limitations to most studies included evaluating subjective symptoms, using different methods of measurement, studying heterogeneous populations, and establishing different TSH thresholds to distinguish between overt and subclinical hypothyroidism. This creates a bias that makes it difficult to compare studies.

All the studies on overt hypothyroidism and a lot of those on subclinical hypothyroidism were done on a small number of people. More studies on larger populations are required to confirm their findings.

In the case of subclinical hypothyroidism, the studies on both its association with brain fog and possible improvement with thyroid hormone replacement therapies showed contradictory results.


Hypothyroidism is when your thyroid gland doesn’t make enough thyroid hormones.

Though the causes of hypothyroidism are diverse, brain fog is a common symptom. Most people experience forgetfulness, lack of focus, and learning difficulties. That’s because oxidative stress and inflammation underlie both hypothyroidism and brain fog in most cases.

To fix brain fog from hypothyroidism, first make sure to get moderate exercise, improve your sleep, and de-stress. These steps will stimulate your thyroid to make thyroid hormones while protecting your brain.

Additionally, check for gluten intolerance, insulin resistance, microbiome imbalances, and gut inflammation. These factors can worsen autoimmune hypothyroidism and keep re-triggering your brain fog even after you heal your thyroid.

Eat a nutritious diet (or supplement to prevent nutrient deficiency) and avoid eating iodine-binding foods like cabbage in large amounts.

About the Author

Carlos Tello, PhD (Molecular Biology)

PhD (Molecular Biology)

Carlos received his PhD and MS from the Universidad de Sevilla.

Carlos spent 9 years in the laboratory investigating mineral transport in plants. He then started working as a freelancer, mainly in science writing, editing, and consulting. Carlos is passionate about learning the mechanisms behind biological processes and communicating science to both academic and non-academic audiences. He strongly believes that scientific literacy is crucial to maintain a healthy lifestyle and avoid falling for scams.

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