What to explain
Explain when your thinking changes and whether a glucose reading was taken at the same time.
I would like to discuss whether low or high glucose, diabetes treatment, or another health problem could be causing my brain fog. Could we review the timing and any glucose reading taken during symptoms, then decide whether I need A1C, vitamin B12, kidney, thyroid, anemia, or sleep checks? If I do not have diabetes, which standard test should we use?
Questions to take in
Ask which problem is being checked and what result would change what happens next.
- Are we checking whether I have diabetes, whether my diabetes treatment needs review, or whether a different condition explains why my thinking is poor?
- What do my A1C and glucose results show, and do I need a repeat test to confirm a diagnosis?
- Could insulin, a sulfonylurea, metformin, another diabetes medicine, steroids, alcohol, or missed meals be contributing to these episodes?
- Would my existing meter or CGM report answer the question, or would more monitoring add cost without changing care?
- Does long-term metformin use, anemia, numbness, balance trouble, or memory change make a vitamin B12 test useful?
- Are my eGFR and urine albumin results up to date, and could kidney disease or a medicine dose be contributing?
- Which other causes of brain fog should we check if glucose does not explain it?
- What is my personal plan for low glucose, sick days, ketones, very high glucose, driving, and when to seek emergency help?
Blood sugar, medicine, and complication checks
What each blood test, monitoring report, medicine review, and kidney check can show.
The right check depends on whether you might have undiagnosed diabetes, already have diabetes, or may be having glucose lows or highs.
HbA1c
A1C estimates average glucose over about three months. For nonpregnant adults, 5.7% to 6.4% is the ADA prediabetes range and 6.5% or higher is the diabetes range. Diagnosis usually needs confirmation when there are no classic symptoms or crisis.
Read the test guideFasting plasma glucose
Fasting plasma glucose measures one morning value after at least eight hours without calories. For nonpregnant adults, 100 to 125 mg/dL is the ADA prediabetes range and 126 mg/dL or higher is the diabetes range. An abnormal result usually needs confirmation.
Ask your doctorBlood glucose meter or CGM review
An existing meter or CGM report can show readings, timing, and low or high episodes during diabetes management. It can't diagnose diabetes or prove that one reading caused the thinking problem.
Ask your doctorMedicine review
A medicine review compares the timing of symptoms with insulin, sulfonylureas, metformin, GLP-1 medicines, SGLT2 medicines, steroids, alcohol, missed doses, kidney function, illness, meals, and activity.
Ask your doctorVitamin B12
Vitamin B12 testing may be useful during long-term metformin treatment, especially when anemia, numbness, balance trouble, or memory change is present. Metformin raises risk; it does not make every user deficient.
Read the test guideKidney function tests
Creatinine with eGFR estimates kidney filtering. A urine albumin-to-creatinine ratio checks for albumin leakage. Kidney disease can affect health, medicine choice, and low-glucose risk.
Read the test guideBefore the appointment
Bring the results you already have and the exact timing of difficult episodes.
Bring your diabetes type, year of diagnosis, recent A1C, fasting or random glucose, oral glucose tolerance test, and earlier results if you have them.
If you already use a meter or CGM, bring the report with dates and times. Mark the readings taken when you felt muddled, shaky, sweaty, thirsty, faint, or unusually tired.
Bring every medicine and supplement, with the dose and time taken. Include insulin, sulfonylureas, metformin, GLP-1 medicines, SGLT2 medicines, and steroids when relevant.
For each suspected low or high, note the reading, symptoms, meal timing, activity, alcohol, illness, medicine timing, what you did, and how long recovery took.
Bring any kidney, urine albumin, vitamin B12, thyroid, CBC, iron, sleep, pregnancy, or infection results that may help explain the same symptoms.
Bring two real examples of what poor thinking stopped you from doing, like driving safely, following a recipe, taking medicine correctly, working, or caring for someone.
Existing readings with dates and symptoms are useful. Standard laboratory tests diagnose diabetes, and your care team should decide whether you need more monitoring.
How the doctor assesses this
Signs that glucose or diabetes treatment may be affecting thinking
- A glucose reading was low or very high while you had brain fog, confusion, shakiness, sweating, thirst, faintness, or unusual tiredness.
- Thirst, frequent urination, blurred vision, weight loss, slow healing, repeated infections, or marked tiredness may happen with the thinking problem.
- Episodes follow insulin or a medicine that can cause low glucose, a delayed meal, illness, alcohol, or a change in activity.
Reasons to check sleep, dehydration, medicines, vitamin B12, thyroid, anemia, kidney disease, infection, stroke, or another neurological problem too
- Thinking stays poor when glucose readings are within the range agreed with your care team and there are no other symptoms of low or high glucose.
- The problem began after a sleep change, infection, medicine change, heavy bleeding, poor food intake, dehydration, or another event that explains the timing better.
- Concentration or memory is getting steadily worse instead of clearing after a brief low or high glucose episode.
- Sudden speech trouble, one-sided weakness, fainting, a seizure, or severe confusion needs urgent medical assessment, even if diabetes seems the cause.
What to understand before choosing care
Decisions to make about diagnosis, treatment safety, other causes, emergency planning, and follow-up.
- Decide whether the visit will check for undiagnosed diabetes, review known diabetes, or look into possible glucose lows or highs.
- If glucose is low or very high during symptoms, identify the medicine, missed meal, illness, activity, alcohol, or other event that may have contributed.
- If A1C and glucose results do not explain the cognitive problem, check sleep, dehydration, medicines, vitamin B12, thyroid, anemia, kidney disease, infection, and neurological causes.
- Use a meter or CGM only when it answers a defined care question.
- Agree what result, symptom, or daily task to review at follow-up.
What the research found
What current guidance and recent studies say about glucose results and thinking problems.
For nonpregnant adults, ADA 2026 uses A1C of 6.5% or higher or fasting plasma glucose of 126 mg/dL or higher. It also uses two-hour glucose of 200 mg/dL or higher, or random glucose of 200 mg/dL or higher with classic symptoms. Without clear symptoms or a crisis, an abnormal result usually needs confirmation.
A1C reflects average glucose over about three months. Anemia, pregnancy, some hemoglobin variants, recent blood loss, dialysis, and other changes in red blood cells can make it misleading.
ADA 2026 states that there is not enough evidence to use CGM to screen for or diagnose prediabetes or diabetes. A CGM report can help manage known diabetes or selected low-glucose risk.
A 2023 review combined nine studies with 1,263 people with type 2 diabetes. The studies linked larger short-term glucose changes with lower thinking-test scores. But they don't prove a glucose change caused one person's thinking problems, or that steadier glucose will fix them.
A 2025 review included 30 studies and 10,469 people with type 2 diabetes. It linked cognitive impairment with age, longer diabetes duration, blood pressure, cardiovascular disease, and higher A1C, but most studies were from China and were not treatment trials.
How diabetes testing and safety change with age, pregnancy, and other risks.
Children and teenagers can develop type 1 or type 2 diabetes. New thirst, frequent urination, weight loss, tiredness, vomiting, stomach pain, or fast breathing needs prompt medical assessment.
For adults without known diabetes, the 2026 ADA guidance starts routine screening at age 35 and earlier when risk factors are present. Symptoms or pregnancy can change when someone needs testing.
Pregnancy uses its own testing schedule and cutoffs. People with prior gestational diabetes need continued testing after pregnancy because later type 2 diabetes risk is higher.
Older adults have a higher risk of severe low glucose, medicine side effects, kidney changes, falls, and difficulty managing a complex plan. Glucose goals may need to be less strict when treatment harms are greater.
Outside pregnancy, men and women share the same diagnostic cutoffs. Pregnancy, past gestational diabetes, polycystic ovary syndrome, blood loss from periods, and anemia can change which test fits or how to read A1C.
If the answer is no
If your doctor will not order every diabetes test
The test choice depends on the type of diabetes being considered, symptoms, risk factors and previous results. CDC describes A1C, fasting glucose, glucose-tolerance and random glucose tests as answering different questions, with autoantibody testing for suspected type 1 diabetes. More tests are not automatically more informative.
What changes the answer
- Name why you need diabetes testing. Screening for type 2 diabetes, investigating sudden symptoms, checking pregnancy-related diabetes and distinguishing type 1 from type 2 require different tests.
- Bring the result that raised concern. Include the date, whether you had eaten, any illness or medicine change and the laboratory report. A number copied into a note isn't enough.
- Explain why A1C may be hard to read. Tell the doctor about anemia, blood loss, a transfusion, kidney or liver disease, pregnancy, or a blood disorder. CDC says these can change how accurate A1C is.
- Ask whether confirmation or monitoring comes next. A screening result, a diagnosis and routine follow-up are different decisions. Request the test and timing that answer the current one.
United States, United Kingdom, and Australia
Who to contact about Diabetes and Brain Fog.
US United States
Book a diabetes and brain-fog review. Bring symptom timing, standard laboratory results, existing meter or CGM data, medicines, low or high glucose episodes, and two examples of tasks that became unsafe or difficult.
- A1C, fasting plasma glucose, a two-hour oral glucose tolerance test, or random plasma glucose with classic symptoms can diagnose diabetes. A result usually needs confirmation when there is no clear crisis.
- CGM is not recommended as a screening or diagnostic test for diabetes. Use it for a defined management reason after discussing whether the result will change care.
- A personal glucose or A1C goal depends on age, pregnancy, medicines, low-glucose risk, other illnesses, daily function, and treatment burden.
UK United Kingdom
Ask for a diabetes review. Bring HbA1c, existing glucose records, medicines, low or high episodes, kidney and B12 results, and exact examples of how thinking or safety changed.
- NICE updated guideline NG28 in February 2026. It says an HbA1c goal should be agreed with the person and relaxed when low glucose, frailty, other illnesses, or treatment burden make stricter control unsafe.
- Finger-prick monitoring isn't routinely offered to every adult with type 2 diabetes. It's for people who take insulin or certain medicines, may have low glucose, are pregnant or ill, or have another defined reason.
- Some adults on insulin for type 2 diabetes can get CGM, including for repeated or severe low glucose or trouble noticing lows.
AU Australia
Book a GP or diabetes-team review. Bring standard laboratory results, existing meter or CGM data, medicines, low or high episodes, and exact examples of how poor thinking affects daily safety.
- Healthdirect considers a blood glucose level below 4 mmol/L to be hypoglycaemia. Confusion, slurred speech, a seizure, or unconsciousness can occur as the level falls.
- Review frequent or severe lows with a doctor or diabetes nurse to deal with the cause and change the diabetes plan.
- Someone who's unconscious or too drowsy to swallow needs emergency help. Call triple zero, and give no food or drink by mouth.
Safety
Show how it affects daily life
- The time of day it lands and the task that got harder, alongside meals, medicines, activity, sleep and alcohol. A glucose reading only if you have a safe reason to check.
- Keep the original meter or CGM time stamp. Use the time stamp, not memory, and copy the usual readings too.
- Record thirst, frequent urination, blurred vision, shakiness, sweating, fast heartbeat, hunger, headache, weakness, or confusion when they occur.
- Keep to your usual medicines, meals and activity, so the readings for this handout come from normal days.
- Follow your existing low-glucose and sick-day plan. Ask the diabetes team for a written plan if you do not have one.
- At follow-up, compare readings with real tasks and symptoms. A number without the timing cannot show whether it caused the thinking problem.
Source checked
Sources behind this handout.
- 01
American Diabetes Association. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes. 2026.
Source - 02
American Diabetes Association. Glycemic Goals, Hypoglycemia, and Hyperglycemic Crises: Standards of Care in Diabetes. 2026.
Source - 03
American Diabetes Association. Pharmacologic Approaches to Glycemic Treatment: Standards of Care in Diabetes. 2026.
Source - 04
National Institute of Diabetes and Digestive and Kidney Diseases. Diabetes Tests and Diagnosis. Current patient guidance.
Source - 05
Chi H et al. Acute glucose changes and cognitive performance in type 2 diabetes. PLOS ONE. 2023. PMID: 37656693.
Source - 06
Zhao Y et al. Risk factors for mild cognitive impairment in type 2 diabetes. Frontiers in Endocrinology. 2025. PMID: 40589509.
Source