When persistent brain fog needs a broader medical evaluation
Other health problems that can feed brain fog, and when extra testing makes sense
Imagine a 45 year old man six months after a highway crash. His headaches have faded, but his thinking still feels slow. He is tired all the time, has gained weight, feels cold when others are comfortable, and has lost interest in sex. He wonders whether this is still the concussion or something else.
Brain fog, the hazy, slow feeling that makes it hard to think clearly and remember, is a common concussion symptom [1]. For most people, symptoms ease within two weeks or so [1]. When the fog lasts for months, the brain injury may still be part of the story. Other health problems can also add to it. Some of them are treatable.
This article covers the contributors worth checking, the less common hormone problems that can follow a TBI, and who may need targeted tests. It also explains why guidelines do not recommend testing everyone.
Why brain fog can have more than one cause
After a concussion, thinking problems can come from the injury itself and from other conditions that pile on top of it. The 2021 VA/DoD guideline for mild traumatic brain injury (TBI), written for veterans and service members, notes that depression, chronic pain and sleep problems often travel with mild TBI [2]. It asks providers to tell patients how these other conditions, and side effects of medicines, may worsen thinking [2].
Some causes of memory trouble can be reversed with treatment, including an underactive thyroid and low vitamin B12 [3]. Depression can cause memory loss that looks a lot like dementia [3]. Finding these causes matters because they call for different care.
When new symptoms appear later
Timing can be a clue. Some symptoms first show up only after the first 30 days following a mild TBI. For those, the VA/DoD guideline suggests an evaluation aimed at that specific symptom, looking for causes other than the brain injury [2]. A new problem months later should not be blamed on the concussion without a check.
The Living Concussion Guidelines, which come from Canada and are revised as studies are published, take a similar view of lasting fatigue. They advise looking for other causes such as mood problems, sleep problems, metabolic disease and imbalances in body salts like sodium [4]. If fatigue continues past one month, they suggest a referral to a specialized concussion service [4].
Start with the most common contributors
Many of the usual contributors are everyday problems that are easy to overlook after a crash.
Sleep is one. Poor sleep and sleep apnea, a condition in which breathing stops and starts during sleep, can both cause fatigue [5]. The VA/DoD guideline calls a look at sleep problems and their causes an essential part of the workup [2].
Mood is another. The TBI Model Systems are a federally supported brain injury research program. They report that about 27% of people with TBI meet the criteria for major depression or a milder form that lasts a long time [6]. Depression commonly brings trouble concentrating and low energy [6]. A provider may use a short questionnaire such as the PHQ-9 to check for it [7, 8].
Medicines are a third. Memory trouble that starts after a new medicine or a higher dose deserves a closer look [8]. The VA/DoD guideline asks providers to review all medicines and supplements for side effects [2].
These problems also affect one another. The Living Concussion Guidelines note that better sleep, pain control and correcting body chemistry problems may lift low mood [7].
Pain and alcohol belong on the list
Chronic pain is one of the conditions that often come with mild TBI [2]. Headaches are on the list of problems that can cause or worsen fatigue [5]. The VA/DoD guideline adds that substance use and unhealthy habits can feed fatigue too [2]. These topics have their own articles in this series, but a broad evaluation should ask about them. Be open with your provider about how much you drink, since the answer can change which causes they look for first.
Comparison compiled by Safety Behind the Wheel Foundation
Possible contributors to lasting brain fog
| Contributor | Clues that may point to it | How it is often checked |
|---|---|---|
| Poor sleep or sleep apnea | Unrefreshing sleep, daytime sleepiness | Sleep history, then a search for the cause |
| Depression | Low mood, low energy, poor focus | Exam and short questionnaires |
| Medicine side effects | Fog after a new medicine or higher dose | Full medicine and supplement review |
| Underactive thyroid | Tiredness, weight gain, feeling cold, forgetfulness | TSH blood test |
| Low vitamin B12 | Tingling hands or feet, tiredness, confusion | Routine blood tests, then B12 level |
| Pituitary hormone problems | Fatigue, low mood, poor focus, low sex drive | Hormone tests ordered by a specialist |
Sources: VA/DoD guideline [2]; MSD Manual [3, 8, 9, 10]; TBI Model Systems [5, 6]; TBI Center of Excellence [11, 12].
Thyroid, vitamin B12 and anemia
These problems are common in the general population. They can look like the lingering effects of a concussion. None of them is specific to brain injury.
An underactive thyroid
Hypothyroidism means the thyroid gland makes too little hormone. Its symptoms are subtle and come on slowly. They can be mistaken for depression, especially in older adults [9]. Many people feel tired, gain weight, become constipated and cannot tolerate cold [9]. Some, especially older adults, seem confused or forgetful [9]. It is common, affecting about 10% of older women [9]. A single blood test called TSH usually makes the diagnosis [9].
Low vitamin B12
Vitamin B12 deficiency can cause anemia, a low red blood cell count, which brings paleness, weakness and fatigue [10]. It can also cause tingling or numbness in the hands and feet. Some people become confused, irritable or mildly depressed [10]. Low stomach acid, which is common in older adults, raises the risk [10]. So do a vegan diet without supplements, weight loss surgery, and some medicines such as antacids and metformin [10].
The deficiency is often first suspected when routine blood tests show unusually large red blood cells, and then the B12 level is measured [10]. Treatment usually clears the anemia in about six weeks [10]. Nerve symptoms that last for months or years may become permanent, which is one reason not to ignore them [10].
Anemia from other causes
Anemia from any cause can lead to fatigue. The TBI Model Systems list it among the conditions that can cause or worsen tiredness after a TBI [5]. Their list also includes an underactive thyroid, low vitamin levels, and heart or lung problems [5]. You can ask your doctor whether any blood tests could help explain your fatigue [5].
Hormone problems after a brain injury
The pituitary is a small gland that sends signals to other hormone glands, including the thyroid [9]. A blow to the head can sometimes affect it. When the pituitary does not make enough of one or more hormones, the condition is called hypopituitarism.
How common it is
Estimates vary widely. A major 2015 review found that most studies report hypopituitarism in 15% to 50% of people with TBI [13]. When people were tested more than once, the share with lasting problems fell to about 12% [13]. A companion review by the same lead author reported a pooled estimate of 27.5% from earlier studies [14]. It also said the risk after mild TBI may be substantial, close to that after moderate TBI [14].
The numbers vary for several reasons, including how severe the injury was, what kind of injury it was, and how long after the injury people were tested [14]. The military’s Traumatic Brain Injury Center of Excellence is more cautious. Its 2022 fact sheet says the rate after concussion alone is not well defined, and that the problem occurs in a small subset of patients [11].
Which hormones and what it can feel like
Growth hormone is the one most often lost after TBI [13]. Next come the hormone that drives the adrenal glands, the hormones that control the testes and ovaries, and the hormone that controls the thyroid [13]. Problems with the adrenal and thyroid signals are uncommon, found in fewer than 10% of these cases [12].
Many symptoms overlap with concussion, including fatigue, low mood and trouble concentrating [11]. The military guidance describes these clusters [12]:
- Low growth hormone: loss of muscle and strength, more fat around the waist, weight gain.
- Low sex hormones: low sex drive, fertility problems, anemia, hair loss, weaker muscles.
- Low adrenal hormones: low blood pressure, weight loss, a general sick feeling and fatigue.
- Low thyroid hormone: weight gain, feeling cold, trouble with short term memory, dry skin.
Because the signs can be mild or subtle, some cases may go unnoticed [14]. These hormone problems may slow recovery and affect thinking [13]. Whether treating them improves function is still unclear [13]. Evidence for treating them after mild TBI is limited [11].
Who may need targeted testing
Guidance differs somewhat between groups, but it agrees on the main points. Hormone tests are not routine for everyone after a concussion. For most people they are not done in the first three months [11].
The military TBI Center of Excellence advises against ordering these tests routinely as a screening tool [12]. It suggests considering a hormone problem when symptoms that fit one last more than three months, or start new up to 36 months after the injury [12]. People with lasting symptoms, risk factors and trouble with daily function should be referred to a specialist such as an endocrinologist, a doctor who treats hormone disorders [11]. Risk factors it names include blast injuries, repeated low level blast exposure and several past TBIs [11].
The 2015 clinical review, which is not limited to military patients, also argues against testing everyone. Its authors say screening every person with mild TBI is not practical or cost effective [14]. They recommend screening people with moderate or severe TBI, and people whose mild TBI was more complicated [14]. Anyone who needed intensive care should be checked early, during that hospital stay [14]. Some people go home from the emergency department without passing out, or with under 30 minutes of posttraumatic amnesia (the confused period when new memories do not stick). The authors say those people do not need pituitary screening [14]. Anyone with signs that suggest a hormone problem can be tested at any time [14].
Checklist from Safety Behind the Wheel Foundation
- Your fog, fatigue or low mood has lasted more than three months, or started new after the first month.
- You have symptoms from the clusters above, such as weight change, feeling cold, low sex drive or muscle loss.
- Your injury was moderate or severe, or you have had several brain injuries or blast exposure.
- You have tingling or numbness in your hands or feet, or you take metformin or antacids long term.
- You snore loudly or feel very sleepy during the day.
- You started a new medicine or a higher dose before the fog got worse.
If several of these fit, bring the list to your next appointment and ask whether a broader evaluation makes sense.
Why testing everyone is not the answer
Guidelines do not support running every test on everyone. The VA/DoD guideline suggests against using brain imaging, blood biomarkers or brain wave tests to diagnose mild TBI [2]. It also suggests against computer based screening tests for routine care [2]. For fatigue, lab tests are suggested when a person’s history points to a medical cause [2].
Hormone tests are tricky to read. They involve both standard blood tests and challenge tests. A specialist should order and interpret them [11]. Results must be compared with the reference ranges of the lab that ran them [12]. In the 2015 review, the share of people with lasting hormone problems dropped sharply when tests were repeated [13]. A single abnormal result early on may not mean a lasting problem.
Hormone screening is meant to be one piece of a full evaluation by a qualified provider [12]. Your symptoms and history are what point that evaluation in the right direction.
Brain fog, sleepiness and driving
If an evaluation finds sleep apnea, let your care team know if you drive. Sleep apnea and some medicines can cause drowsiness. Drowsiness slows reaction time and weakens attention and judgment [15]. Driving rules for medical conditions vary by state.
Signs that need urgent care
Some symptoms should not wait for a scheduled visit. After a head injury, these danger signs call for 911 or a fast trip to the emergency department [1]:
- Severe drowsiness, fainting, or inability to wake up.
- Confusion, restlessness, agitation, or not knowing people or places.
- A headache that worsens and will not let up.
- Repeated vomiting.
- Slurred speech or unusual behavior.
- Weakness, numbness, poor coordination, convulsions or seizures.
- One pupil larger than the other.
Get prompt medical care for sudden severe confusion or new nervous system symptoms [3]. A problem with the adrenal hormones can be life threatening [14], so seek care quickly for severe weakness with very low blood pressure [12]. If you feel depressed and have thoughts of harming yourself, call 988 or 911 [3].
Questions to ask your care team
Question checklist from Safety Behind the Wheel Foundation
- Could something other than the concussion be adding to my brain fog?
- Have my sleep, mood, pain and medicines been checked as possible causes?
- Do my symptoms suggest any blood tests, such as thyroid, vitamin B12 or a blood count?
- Should I be checked for sleep apnea?
- Do my injury and symptoms suggest a pituitary hormone problem?
- If hormone tests are needed, who should order and read them?
- If my first tests are normal, when should we look again?
- What changes in my symptoms would mean I should call before my next visit?
- Is it safe for me to drive while we sort this out?
Frequently asked questions
Should I ask for a full panel of blood tests?
Guidelines favor tests that fit your history and symptoms over a long panel for everyone [2, 12]. It is still fair to ask whether any blood tests could help explain your symptoms [5].
Can a mild concussion really affect my hormones?
It can in some people. One review found the risk after mild TBI may be close to that after moderate TBI [14]. But military guidance says the true rate after concussion alone is not well defined [11]. Most people with a mild TBI do not need hormone testing unless symptoms point that way [14].
If my hormone tests are normal now, could a problem show up later?
Possibly. Military guidance notes that these problems can start any time up to 36 months after the injury [12]. The 2015 review suggests a full evaluation about 12 months after injury for those at risk, with testing any time symptoms appear [14].
The bottom line
Recommendation from Safety Behind the Wheel Foundation
When brain fog lasts for months after a concussion, the injury may not be the whole story. Start with the common contributors, such as sleep, mood, pain and medicines. Then ask whether your symptoms point to a thyroid problem, low vitamin B12, anemia, sleep apnea or, less often, a pituitary hormone problem.
Guidelines favor targeted testing. Share your full symptom history with your care team, ask which tests fit your situation, and get urgent care for any danger signs.
Sources and further reading
- 1Centers for Disease Control and Prevention. Symptoms of mild TBI and concussion. Last reviewed September 2025. cdc.gov/…/index.html
- 2U.S. Department of Veterans Affairs, U.S. Department of Defense. VA/DoD clinical practice guideline for the management and rehabilitation of post-acute mild traumatic brain injury: provider summary. Version 3.0. 2021. healthquality.va.gov/…/VADoDmTBICPGProviderSummary…
- 3Freedman M. Memory loss. MSD Manual Consumer Version. Updated January 2026. msdmanuals.com/…/memory-loss
- 4Living Concussion Guidelines. Section 11: Fatigue (adult guideline). Ottawa Hospital Research Institute. Updated May 2023. concussionsontario.org/…/fatigue
- 5Model Systems Knowledge Translation Center. Fatigue and traumatic brain injury. Updated 2024. msktc.org/…/fatigue-and-traumatic-brain…
- 6Model Systems Knowledge Translation Center. Depression after traumatic brain injury. 2025. msktc.org/…/depression-after-traumatic-…
- 7Living Concussion Guidelines. Section 8: Mental health disorders (adult guideline). Ottawa Hospital Research Institute. Updated April 2023. concussionsontario.org/…/mental-health-disorders
- 8MSD Manual Consumer Version. Table: some causes and features of memory loss. 2026. msdmanuals.com/…/some-causes-and-features-of…
- 9Boucai L. Hypothyroidism. MSD Manual Consumer Version. Updated May 2026. msdmanuals.com/…/hypothyroidism
- 10Johnson LE. Vitamin B12 deficiency. MSD Manual Consumer Version. Updated June 2026. msdmanuals.com/…/vitamin-b12-deficiency
- 11Traumatic Brain Injury Center of Excellence, Defense Health Agency. Neuroendocrine dysfunction (NED) following concussion/mild TBI (mTBI): information for the primary care manager. August 2022. health.mil/…/Neuroendocrine-Dysfunction-…
- 12Traumatic Brain Injury Center of Excellence. Indications and conditions for neuroendocrine dysfunction screening post mild traumatic brain injury. Reviewed April 2021. health.mil/…/Indications-and-Conditions-…
- 13Tanriverdi F, Schneider HJ, Aimaretti G, Masel BE, Casanueva FF, Kelestimur F. Pituitary dysfunction after traumatic brain injury: a clinical and pathophysiological approach. Endocrine Reviews. 2015;36(3):305-342. doi.org/10.1210/er.2014-1065
- 14Tanriverdi F, Kelestimur F. Pituitary dysfunction following traumatic brain injury: clinical perspectives. Neuropsychiatric Disease and Treatment. 2015;11:1835-1843. doi.org/10.2147/NDT.S65814
- 15National Highway Traffic Safety Administration. NHTSA drowsy driving research and program plan (DOT HS 812 252). March 2016. nhtsa.gov/…/nhtsa-drowsy-driving-resear…
This article reflects the safety priorities of concussion recovery and road safety education. It is meant for general education and is not medical advice. Testing decisions depend on each person’s history, so check official sources and talk with your own care team about your symptoms. Reviewed October 2026.
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