Most people call it “brain fog,” but what they mean is inability to remember words, poor short-term memory, and difficulty concentrating. I see it all the time, and there are a number of possible causes. Usually the rest of the clinical picture clarifies the underlying cause, or at least gives me an idea what kind of testing is in order. Here are some of the most common possibilities.

Candida or Yeast Overgrowth

Your gut has its own microscopic ecosystem. About 85% of it ought to be “good” bacteria, but there are trace amounts of other organisms as well. Candida (and other forms of yeast) are some of them. These single-celled fungi eat sugar.  I tend to think of this when people tell me they’re addicted to sugar, or when a diet diary shows me they’re eating either a lot of sugar or white carbs. As a byproduct of its sugar metabolism, yeast produces acetaldehyde – the same toxic byproduct your liver produces when processing alcohol. (Acetaldehyde is actually the chemical responsible for hangovers.)  So effectively, candida can make you feel a little bit drunk!  People usually describe it as poor memory, fuzzy thinking, or poor word recall. 

Adrenal Fatigue

Your adrenals are these two pyramid-shaped glands that sit on top of your kidneys. They’ve got several jobs, but the biggest is to help your body cope with stress. When a stressful event occurs, the core of your adrenals flood your body with adrenaline. When the stressful event subsides, the outside (or the cortex) of the adrenals pump out a matching amount of cortisol to counterbalance it. 

If you’re chronically stressed and your adrenal cortex can’t keep up with the demand for either adrenaline (see the catecholamine section below) or cortisol, you’ll feel apathetic, and all you’ll want to do is stare at the wall. Of course, this also impairs concentration and memory. Adrenal fatigue moves up higher on my list of differentials if the patient also reports high stress, fatigue, or poor stress tolerance.


There’s a big overlap between adrenal fatigue and hypothyroidism: specifically, too much cortisol inhibits the conversion of inactive to active thyroid hormone. But hypothyroidism may occur independent of adrenal fatigue, as well. Here’s how it works.

Your pituitary produces the hormone TSH (Thyroid Stimulating Hormone).  It tells your thyroid to produce the less active thyroid hormone T4 , and T4 then travels to your peripheral tissues (primarily the gut and liver), where it gets converted to T3. T3 is heavily involved in controlling your metabolism.

T4 also gets converted to T3 in the prefrontal cortex of the brain—the part primarily responsible for decision making and executive function. This may be a big part of why one of the symptoms of hypothyroidism is difficulty concentrating. I’m more likely to suspect hypothyroidism as the cause if the patient also has other confirmatory symptoms, such as unexplained weight gain, fatigue, constipation, or hair loss.

Insomnia or Insufficient Sleep

According to the CDC, some 40% of Americans are chronically sleep deprived—defined as unintentionally nodding off at least once during the previous month.

Why does sleep deprivation cause memory problems? One possible (chilling) explanation is that sleep gives your brain time to “clean up” toxic proteins that can accumulate throughout waking hours, including beta amyloid and tau—the proteins linked with Alzheimer’s Disease. In this study, mice who were only allowed to sleep four hours per night were not only impaired in their ability to recall and learn new tasks; they were also found to have an increased level of tau in their brains. 

Ruling this cause in or out generally requires a few questions about sleep habits and sleep hygiene.

Food Allergies

Both IgE and IgG food allergies/sensitivities release inflammatory cytokines, which can have a systemic effect. According to this article, inflammatory cytokines can set off a process in the brain causing focal inflammation.

This means that anything causing systemic inflammation can likely be the cause… but food reactions (particularly IgG, which are by far the most common type) are one of the most frequent sources of inflammation I see.

Sex Hormone Imbalance

There’s a big connection between sex hormones and neurotransmitters, particularly the catecholamines (dopamine, norepinephrine and epinephrine) responsible for focus. Specifically, estrogen decreases the functioning of the enzyme MAO, which breaks down the catecholamines, while progesterone increases MAO functioning.

Generally we talk about estrogen dominance relative to progesterone as causing problems. But if your estrogen is too low, these neurotransmitters will be too—and you’ll have a hard time focusing.

Low Catecholamines

As mentioned above: catecholamines, and especially dopamine, is responsible for focus (as well as keeping your mood elevated, hopeful, and excited.) While acute stress raises dopamine levels in the prefrontal cortex, chronic stress depletes it. This is in part because the mechanisms that break dopamine down get upregulated (increased) when there’s a lot of dopamine floating around—such as when you’ve been stressed out for awhile. Dopamine transporters also decrease in chronic stress.  It also doesn’t help that dopamine is the precursor to adrenaline: the first hormone released in a “fight-or-flight” situation. What dopamine you might have had gets used up, and the extra dopamine released in response to high cortisol levels gets broken down faster, too. I’m likely to suspect this cause if symptoms follow a period of high stress.

Another possible cause of low catecholamines is low blood sugar. Low blood sugar means low insulin, because insulin allows amino acids from the protein in our diet, such as dopamine precursors (phenylalanine and tyrosine) to cross the Blood Brain Barrier (BBB). When insulin is too low, this doesn’t happen efficiently, and you can’t get the dopamine precursors into the brain. I’m likely to suspect this cause if symptoms are worse between meals, and accompanied by shakiness or irritability, and resolved or improved with food.

Hypothyroidism (see above) can also cause low dopamine. This is because the hypothalamus produces TRH, which tells the pituitary to make TSH, which tells the thyroid to make T4. Healthy TRH levels also stimulate the release of dopamine, suggesting that the opposite may also be true: low thyroid may also lead to low dopamine, and many studies suggest that this is the case. Additionally, dopamine and thyroid hormone also share an amino acid precursor: Tyrosine. If tyrosine is low, there likely won’t be enough to go around for both thyroid hormone and dopamine.

The Upshot

“Brain fog” has a lot of possible causes. Sometimes it’s not just one, but several of the causes mentioned above.