Hypothyroidism is a hormone imbalance with strong effects in most organs of the body. “Brain fog” is not a medical condition. It’s a term used for a group of symptoms that can affect your cognition, mood, and energy. Read on to learn more about the possible link between hypothyroidism and “brain fog”.

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 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 may 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 small studies [19, 20].

Brain Fog and 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” may result from many causes, but nearly 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 more.

Overt Hypothyroidism

This condition is linked to “brain fog” symptoms, especially in elderly people, as seen in 6 observational studies on over 200 people. The main finding 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 small 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 and inconclusive. 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 may manifest as “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+].

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

Euthyroid Sick Syndrome

Euthyroid sick syndrome, 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 result in “brain fog” like symptoms in people undergoing major surgery [89, 90, 84+].

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

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

In another study on over 600 people, euthyroid sick syndrome 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 typically show “brain fog” symptoms, 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].

A future avenue of research involves adding vitamin E to LT4 therapy as some scientists believe it will increase its effectiveness for cognitive symptoms because it protects the brain from free radicals. This combination was better than LT4 alone in rats [124].

Hypothyroidism and Health Risks

Dementia

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 might be inherited together [125].

The association of subclinical hypothyroidism with Alzheimer’s is weak to nonexistent. 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].

Ways to Help Brain Fog Symptoms and Hypothyroidism

Lifestyle Changes

1) Do More Exercise

Always remember to speak with your physician before starting on any new exercise regimen or drastic changes in diet.

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 exercises 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.

Diet

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” symptoms with reduced memory, attention, executive function, and processing speed after eating. In a small trial on 11 people recently diagnosed with celiac disease, 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].

Speak with your physician to see if you suffer from gluten intolerance to determine if it could be a possible cause for your “brain fog” symptoms.

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 may result in “brain fog” symptoms [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 possible ways to combat it.

6) Reduce Goitrogens

Goitrogens are substances that reduce iodine uptake by the thyroid gland, resulting in 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 “brain fog” symptoms.

7) Gut Health

Reduce Inflammation

Gut inflammation produces cytokines that are currently being investigated to determine if they can result in cognitive problems and behavioral changes. Although it’s important to remember the link between mental fatigue and 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. 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 might be worsening your “brain fog” symptoms from hypothyroidism, you can try to follow an elementary diet for 2 weeks and then add one at a time back in.

Gut Microbiome

Some scientists believe that gut microbiota plays a 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. These changes are associated with obesity and cognitive impairment [204+, 205, 206].

While the full link and efficacy remains unclear, recent studies have shown that probiotics improved cognitive function and the gut microbiome composition in some small 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) were associated with cognitive problems and behavioral changes in mice [210, 211, 212, 213, 214, 215].

Interestingly, hypothyroidism is associated with bacterial overgrowth in the small bowel. It would be interesting for future research to determine if this overgrowth may worsen “brain fog” symptoms by disturbing the normal microbiome [216].

Supplements

The following supplements might be of benefit in restoring normal 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 this vitamin reduced fatigue in 3 people with Hashimoto’s hypothyroidism [232, 233], though it’s very important to remember much further work needs to be done to determine how useful Vitamin B1 is in people with this disease.

Remember, 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. Hence, remember to speak with your doctor before taking any supplements.

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” symptoms 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.

Takeaway

Hypothyroidism is when your thyroid gland doesn’t make enough thyroid hormones.” brain fog” is not a medical condition but a term used for a group of symptoms that can affect your cognition, mood, and energy.

Though the causes of hypothyroidism are diverse, it may manifest in “brain fog” symptoms. Most people experience forgetfulness, lack of focus, and learning difficulties. Some researchers believe that oxidative stress and inflammation play a role in these symptoms.

Some of the ways to get symptomatic relief from “brain fog” as a result of hypothyroidism include getting moderate exercise, improving sleep, and lowering stress. These steps are theorized to stimulate the thyroid to make more thyroid hormones improving brain health.

Additionally, gluten intolerance, insulin resistance, microbiome imbalances, and gut inflammation are all being currently investigated to determine their role in these symptoms. These factors may worsen autoimmune hypothyroidism and exacerbate “brain fog”.

Eating a nutritious diet (or supplements to prevent nutrient deficiency) and avoiding eating iodine-binding foods like cabbage in large amounts may help.

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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