Vitamin B1 (Thiamine)

In a nationwide survey of Belgian adults, about one in nine had low blood levels of vitamin B1 (thiamine), measured from finger-prick dried blood samples, though very few had a diet that looked short on it. That survey used a mass-spectrometry method (LC-MS/MS) rather than the HPLC method this test uses, so its cutoffs don't carry over directly. Food records and blood levels can still disagree by that much, which is why it's worth measuring directly.

Thiamine matters most to the organs that burn the most fuel: your heart, brain, and nerves. A real shortage can show up as tingling feet, heart failure, or a sudden confusional state called Wernicke encephalopathy, and the heart problems in particular can reverse quickly once they're treated. This test reads your stores directly, and that's most useful if you eat low-carb, have had weight-loss surgery, live with diabetes, or are pregnant.

What the Test Measures

Your body can't make thiamine, so all of it comes from food. Your cells convert most of it into an active form, ThDP (thiamine diphosphate), which several enzymes need to turn sugar and other fuels into usable energy. Most of the thiamine in your blood is ThDP held inside cells, mostly red blood cells.

That's why this test uses whole blood. Because red cells carry most of the vitamin, a whole-blood level reflects your body's stores, and researchers treat it as a sensitive, specific measure of thiamine status. A plasma or serum test picks up only the small share outside cells, and it mostly reflects what you've eaten recently.

The lab measures ThDP by high-performance liquid chromatography (HPLC). The method separates the molecules in your sample before measuring them, so it can pick out thiamine from everything else. Inflammation doesn't drag whole-blood thiamine down the way it lowers plasma levels of many vitamins, which is why European clinical nutrition guidelines prefer red-cell or whole-blood measures when someone is unwell. It isn't perfectly fixed, though. After knee replacement surgery, whole-blood ThDP rose about 25% during the body's inflammatory response, so a reading taken during acute illness may sit somewhat higher than your usual level.

Heart Failure

Severe deficiency can cause wet beriberi, a kind of heart failure in which the heart pumps more blood than usual and still can't meet the body's needs. A sudden, shock-like version is called Shoshin syndrome. It has been reported in an adult woman on an unbalanced diet and in an overweight teenager, and in both cases heart function recovered after thiamine treatment.

In Kashmir, where deficiency is common, researchers have described heart failure in adults that responds to thiamine and argue it's under-recognized. It's less common in wealthier settings. In a Canadian study of heart failure outpatients, only about 6% were deficient, the same rate as people without heart failure, and long-term diuretic use made no difference. Whether diuretics drain thiamine is still debated across the wider research.

If you have heart failure, a normal result is what most people in your position get. A low one points to a cause you can fix, and since the heart effects of deficiency respond to treatment, it's worth tracking down.

Brain, Nerves, and Memory

Your brain and nerves are among the first tissues to suffer when thiamine runs out. Wernicke encephalopathy causes confusion, unsteady walking, and abnormal eye movements, and it can leave lasting damage if it isn't treated. It isn't limited to heavy drinkers. Reviews describe it after weight-loss surgery, severe vomiting in pregnancy, sudden major weight loss, and cancer, and it has been reported in exclusively breastfed infants in Kashmir.

Glucose given by IV can use up what little thiamine is left in someone already running low, which is why people at risk should get thiamine along with it. Deficiency can also damage the nerves of the hands and feet, causing numbness, tingling, or weakness. That form is known as dry beriberi.

Blood thiamine also differs in Alzheimer's disease. In a large Chinese study, blood ThDP measured by HPLC was lower in people with Alzheimer's and helped tell them apart from people with vascular or frontotemporal dementia. In another group of Alzheimer's patients, lower ThDP went along with poorer nutrition and worse cognitive decline. These are associations, so they don't show that low thiamine causes Alzheimer's, and current diagnostic guidance for Alzheimer's relies on other blood markers.

Diabetes and Kidney Health

People with diabetes tend to run lower. A meta-analysis of studies on thiamine and diabetes found lower levels of several thiamine markers in people with diabetes, and the gap was larger in those who had albumin in their urine, an early sign of kidney damage.

The kidneys seem to be part of the reason. In people with type 2 diabetes, more thiamine escapes into the urine, and that loss was tied to poorer blood sugar control. In a Czech study of people with diabetes, thiamine status was linked to faster progression of diabetic kidney disease and to higher mortality. Most of these studies measured thiamine in plasma or urine, a different measurement from this whole-blood test.

Dialysis brings its own risk. In one Chinese study, about 11.5% of people on maintenance hemodialysis were deficient by whole-blood measurement, and lower thiamine went with worse anemia. Part of that link may be the anemia itself, since fewer red cells pull the whole-blood number down. Other dialysis research finds that enzyme-based tests of how well thiamine is working flag a shortfall more often than blood levels do, so a normal number here doesn't fully settle the question.

If you have diabetes, especially with albumin showing up in your urine, thiamine is worth checking alongside the blood sugar and kidney tests you already track.

Pregnancy and Babies

In Kashmir, more than a third of pregnant women studied had low whole-blood thiamine, and low levels went with poorer outcomes for their babies.

A nationally representative survey of Cambodian women and their children found a similar picture, with babies 6 to 12 months old at highest risk. Breastfed babies depend on the thiamine in breast milk, and milk levels track the mother's own status. Depending on which cutoff the researchers used, anywhere from 27% to 78% of the women counted as deficient, which shows how unsettled the definition of a low level still is.

If you're pregnant or breastfeeding, and especially if you've had severe vomiting or eat a narrow diet, your own level sets your baby's supply.

Acute Illness and Hospital Stays

Being acutely sick is one of the main settings for a shortfall. Among emergency department patients with sepsis, diabetic ketoacidosis (a dangerous acid buildup from uncontrolled diabetes), or cancer emergencies, about one in five were deficient by whole-blood levels. Women, older people, and people with low white blood cell counts were more likely to be low.

In acutely ill cancer patients seen in the emergency department, deficiency was common and rarely treated. It isn't universal in hospitals, though. None of 238 older adults in a German hospital were deficient when tested by HPLC, though the authors noted this may partly reflect the cutoffs used.

Who Usually Tests Normal

Several groups you might expect to run low mostly didn't.

Sources: Pourhassan et al.; Gibson et al.; Gangat et al.

The anorexia finding comes from one small pilot study, and the wider research is mixed. Whole-blood deficiency rates in anorexia have ranged from none to about 15%, and older enzyme-based estimates run as high as 38%.

What this means for you: a diagnosis or a risk label doesn't decide your number. Testing tells you whether a risk has turned into an actual shortfall.

Ancestry also matters. In a U.S. study of people with alcohol use disorder, HIV, or neither, race explained more of the variation in whole-blood ThDP than diagnosis did, and Black participants had lower levels than White participants. What that means for health isn't known yet, and in that study low levels didn't track with nerve damage.

What a High Result Means

High results are much less studied than low ones. Heavy drinking is a classic cause of deficiency, yet a group of Australians with alcohol use disorders had high whole-blood thiamine, and none were in the deficient range. In one Mayo Clinic series of people with myeloproliferative neoplasms, many tested above range, possibly because these cancer cells lean on thiamine-dependent pathways, though that idea is unproven and another cohort found levels mostly within the normal range.

Both findings make sense once you read the number as a measure of your current supply. A risk factor raises the odds that supply runs short, and the test tells you whether it has. None of these studies tied a high whole-blood level to harm. If you take a B-vitamin supplement, write that down next to a high result.

Why a Single Reading Can Fool You

The biggest problem in reading this test is that there's no agreed definition of normal.

• Lab and unit differences: cutoffs vary between labs and studies, and so do the units results are reported in. Compare your number only with the reference interval on your own report, and use the same lab for retests.
• Sample handling: thiamine diphosphate breaks down with light and warmth, so a sample that wasn't protected from light or kept frozen on the way to the lab can read falsely low.
• Red cell count: because most of the measured thiamine sits inside red cells, anemia can pull the whole-blood number down even when each cell carries a normal amount. In a 2026 study, about two-thirds of results flagged low were normal once adjusted for hemoglobin. Some labs report a hemoglobin-adjusted value for this reason; if yours doesn't, check a complete blood count alongside it.
• Acute illness: inflammation doesn't lower this number the way it lowers plasma thiamine, but whole-blood ThDP rose about 25% after major surgery, so a reading during acute illness may sit a little above your usual level.
• Slow response to change: whole-blood ThDP seems to move slowly. In people with extreme anorexia who started in the normal range, 10 days of oral thiamine didn't raise it significantly, so a retest days after starting a supplement may show little.
• Sample type: finger-prick dried blood samples closely matched standard venous whole blood in validation work using LC-MS/MS, a different method from this test, and mailing them didn't change results meaningfully, but switching sample types or methods between tests still adds noise to a trend.

Tracking Your Trend

No study has measured how much whole-blood thiamine varies within one person from week to week. That makes a single reading harder to interpret and puts more weight on repeat tests. Labs judge whether two results truly differ using a reference change value. That's the gap two readings need to clear before the difference exceeds normal biological and lab noise.

Get a baseline, retest about three months after any change in diet, supplements, or medication, then check at least once a year. Use the same lab and method each time. The evidence that supplements raise this marker comes from red-cell ThDP, which accounts for most of what the whole-blood test measures.

The trend matters most after weight-loss surgery, when deficiency rates rise in the first months and then fall, and if you follow a low-carb diet long term.

What to Do With an Unexpected Result

A low result with symptoms is urgent. Confusion, unsteady walking, unusual eye movements, new breathlessness, or swelling in your legs alongside a low level calls for same-day medical care, because Wernicke encephalopathy and thiamine-related heart failure need prompt treatment, often by injection.

A low result without symptoms is a prompt to find the cause. The usual suspects are a low-carb or very restrictive diet, heavy drinking, weight-loss surgery, ongoing vomiting or diarrhea, long-term acid-suppressing medication, and diabetes. Check your HbA1c (average blood sugar over about three months), urine albumin, and kidney function, and add a magnesium level. If you're anemic, keep in mind that anemia alone can lower this number.

Retest once you've dealt with the likely cause. If the level stays low despite a good diet and supplements, absorption becomes the main question, and a gastroenterologist is the right specialist. In children with unexplained neurological illness, a very low whole-blood ThDP can point toward rare inherited disorders of thiamine transport, which genetic testing can confirm.

A high result in someone who feels well rarely needs action. Note any supplements and keep tracking.