Vitamin B1 (Thiamine) Blood Test: Deficiency & Levels

The vitamin B1 (thiamine) blood test checks a vitamin your body needs to turn carbs into energy. Learn normal levels, deficiency signs (beriberi, Wernicke), causes, and when it's measured.

Published July 20, 202610 min readWritten by the Blood Analysis Team · Reviewed and verified by Julien Priour

Vitamin B1, or thiamine, is a water-soluble vitamin your body needs to turn carbohydrates into energy. Your body stores very little of it — only a few weeks' worth — so a deficiency can develop quickly in certain situations (heavy alcohol use, poor nutrition, weight-loss surgery, severe pregnancy vomiting). This guide covers what thiamine does, normal levels, what a low result means, the deficiency diseases (beriberi, Wernicke encephalopathy), and when the test is actually useful. Vitamin B1 is best read as part of a broader vitamin panel, alongside B12, folate, and vitamin D.

Key takeaways

Vitamin B1 (thiamine) is a water-soluble vitamin; its active form, thiamine pyrophosphate (TPP), is a cofactor for carbohydrate metabolism and energy production.¹²
Body stores are small (on the order of 2–3 weeks), so deficiency can appear quickly when intake is low or needs are high.¹
Deficiency causes beriberi ("wet" = heart failure with edema; "dry" = neuropathy) and Wernicke encephalopathy (confusion, unsteady gait, eye-movement problems), which can progress to Korsakoff syndrome (lasting memory loss).¹³
High-risk settings: chronic alcohol use, malnutrition, bariatric surgery, severe pregnancy vomiting (hyperemesis), IV/tube feeding, dialysis, and heart failure treated with diuretics.⁴⁵⁶
⚠️ In an at-risk patient, thiamine must be given BEFORE any glucose (an IV sugar load can trigger Wernicke encephalopathy); the diagnosis is clinical and treatment should not wait for the blood level.⁴³
Vitamin B1 is not a cancer marker; outside of true deficiency risk, taking thiamine "for energy" or "for fatigue" has no proven benefit.²

What is vitamin B1 (thiamine)?

Thiamine is one of the eight B vitamins. Because it is water-soluble, it isn't stored in body fat for long: you keep only small reserves (mainly in muscle, heart, liver, and brain) covering roughly 2–3 weeks. It therefore has to come in regularly from food — whole grains, legumes, pork, yeast, nuts, and seeds. Refining grains and cooking both remove some of it.²

Inside cells, thiamine is converted to its active form, thiamine pyrophosphate (TPP). TPP is a cofactor — an essential helper — for several key enzymes that burn carbohydrates to make energy (notably pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, at the core of energy metabolism). The brain and heart, which use enormous amounts of energy, are the first organs to suffer when thiamine runs short.¹²

B1 is not B12 or B9. Don't confuse vitamin B1 (thiamine) with B12 (cobalamin) or folate (vitamin B9): these are distinct B vitamins with different roles and different deficiency syndromes. This guide is about thiamine only.

Why the test is done

Unlike routine panels, vitamin B1 is not tested by default. Your clinician may order it when there are signs of deficiency or a high-risk setting:⁴³

chronic alcohol use (low intake plus poor absorption) — the leading cause of thiamine deficiency in Western countries;
malnutrition, eating disorders, or frail older adults;
after bariatric surgery (sleeve, gastric bypass), especially with prolonged vomiting;⁵
severe pregnancy vomiting (hyperemesis gravidarum);⁴
prolonged parenteral (IV) nutrition, dialysis, or heart failure treated with diuretics (which flush thiamine out in the urine);⁶
when there are neurological symptoms (confusion, unsteady gait, abnormal eye movements, tingling legs) or unexplained heart failure.⁷

The key point: in urgent situations, the diagnosis of deficiency is primarily clinical. The blood level takes time to come back and must never delay treatment — if Wernicke encephalopathy is suspected, thiamine is given immediately, without waiting for the result.³⁴

How the test is done

The vitamin B1 test is a standard blood draw from a vein. You usually do not need to fast, but follow your lab's exact instructions, since the draw may be bundled with other tests that do require fasting (see Do you need to fast before a blood test?). Because thiamine is light-sensitive, the tube is often shielded from light and sent to the lab quickly. Tell your clinician about any B-vitamin supplements, which can skew the result.²⁸

Normal ranges

Thiamine can be measured two ways: as total thiamine in blood (usually whole blood, because the vitamin is concentrated in red cells) or as erythrocyte transketolase activity (an older functional test). The values below are indicative — they vary widely by lab and method. Always compare against the range printed on your report.

Test	Indicative value (adult)	Unit
Thiamine (whole blood)	~70 – 180 (≈ 20 – 55 µg/L)	nmol/L
Transketolase activity — "TPP effect"	< 15% normal · 15–25% marginal · > 25% deficient	%

Units: results appear in nmol/L or µg/L (ng/mL) depending on the method (HPLC vs. enzyme activity), and lab cutoffs differ. A "normal" level does not fully rule out early deficiency — the clinical picture comes first. Only your clinician can interpret the number in your context.²¹

Low vitamin B1: deficiency and beriberi

A low vitamin B1 means a thiamine deficiency. Depending on how deep and how fast it develops, it can cause:¹⁷⁹

"Wet" beriberi — a heart problem, with breathlessness, leg swelling, a fast heart, and in severe cases high-output heart failure that can improve dramatically with thiamine;⁷
"Dry" beriberi — a nerve problem (peripheral neuropathy), with tingling, weakness, and leg cramps;
Wernicke encephalopathy, a neurological emergency classically combining confusion, unsteady gait (ataxia), and abnormal eye movements (the full triad is actually uncommon);³
if untreated, lasting memory loss — Korsakoff syndrome.¹

The main causes of low thiamine are chronic alcohol use, malnutrition, bariatric surgery, prolonged vomiting (including in pregnancy), unsupplemented IV/tube feeding, dialysis, and long-term diuretics (heart failure).⁴⁵⁶¹⁰

⚠️ Critical safety point. In someone at risk of deficiency, giving glucose (an IV sugar drip) without thiamine can trigger or worsen Wernicke encephalopathy. So the rule is: thiamine BEFORE glucose. This is always a clinician's decision — do not self-treat if you have symptoms; seek care.⁴³

High vitamin B1

A high thiamine level generally carries no worrying meaning. Because vitamin B1 is water-soluble, any excess is cleared in the urine, so overdose from food or oral supplements is very unlikely. A high value usually just reflects recent supplementation. There is no "too much vitamin B1" disease.²

How it fits with the other B vitamins

Thiamine is rarely read alone: a dietary deficiency (alcohol, malnutrition, bariatric surgery) often comes with other vitamin shortfalls. That is why your clinician may measure it alongside B12, folate (B9), and vitamin D, as part of a vitamin panel. Correcting all the deficiencies, not just one, is part of the care.⁵

Vitamin B1 and cancer? Thiamine is not a tumor marker: a low level speaks to nutrition and body stores, not a tumor. And taking B1 "for fatigue" without a proven deficiency has no demonstrated benefit.²

What affects your vitamin B1

Several things change your thiamine status: dietary intake (whole grains, legumes, pork) and cooking losses, alcohol (which lowers both intake and absorption), higher needs (pregnancy, breastfeeding, intense exercise, acute illness, sepsis), urinary losses on diuretics, malabsorption (bariatric surgery, gut disease), and dialysis. Low magnesium can also keep thiamine from working properly. Tell your clinician about your diet, alcohol use, medications, and supplements — they all change the interpretation.⁴⁶²

Recent research

According to recent PubMed-indexed publications:

Wernicke encephalopathy is still under-diagnosed. A 2024 systematic review confirms the diagnosis is frequently missed and IV thiamine underused, for lack of clear diagnostic criteria and guidelines; the authors call for early, generous treatment as soon as it is suspected.³ (Cantu-Weinstein A et al., Gen Hosp Psychiatry, 2024 — DOI.)
After bariatric surgery. A 2024 review highlights the link between thiamine deficiency, obesity, diabetes, and weight-loss surgery, and the role of primary-care clinicians in monitoring and supplementing vitamins after the procedure.⁵
Cardiac beriberi hasn't disappeared. A 2023 study reports high-output heart failure responding to thiamine in adults, noting that the response to treatment can act as an indirect marker of deficiency.⁷ Another showed that furosemide (a heart-failure diuretic) lowers thiamine levels in hospitalized patients.⁶
Thiamine in sepsis / critical care. Pairing vitamin C + thiamine + hydrocortisone in sepsis raised high hopes, but the randomized VICTAS trial (JAMA, 2021) showed no benefit on survival or ventilator-free days — so it is not a routine sepsis treatment.¹¹

These findings concern diagnosis and research; they do not authorize self-medication and do not replace your physician's advice.

Get your vitamin B1 interpreted by AI DiagMe

A vitamin B1 level is never read alone: its meaning depends on your diet, your alcohol use, your medications (diuretics), any digestive surgery, and your other B vitamins — B12, folate, vitamin D. That cross-reading is what gives the result its real value.

👉 AI DiagMe interprets your lab results — blood, urine, or stool — in plain language, taking your whole profile into account. An informational service that does not provide a diagnosis and complements, never replaces, your physician.

Frequently asked questions

What does vitamin B1 (thiamine) do?

It's a water-soluble vitamin whose active form (TPP) works as a cofactor to turn carbohydrates into energy. The brain and heart, which use a lot of energy, depend on it especially. Because body stores are small (2–3 weeks), you need a regular supply.

What is a normal vitamin B1 level?

Indicatively, whole-blood thiamine runs about 70–180 nmol/L (≈ 20–55 µg/L), and the transketolase "TPP effect" is normal below 15%. These values vary by lab and method — read the range on your report.

What does low vitamin B1 mean?

A thiamine deficiency. Depending on severity, it can cause beriberi (cardiac "wet" or neurological "dry") or Wernicke encephalopathy (confusion, unsteady gait, eye-movement problems). Common causes are alcohol, malnutrition, bariatric surgery, and prolonged vomiting.

What are the symptoms of vitamin B1 deficiency?

Fatigue, poor appetite, tingling and weak legs, breathlessness and swelling (cardiac form), and in neurological forms, confusion, trouble walking, and abnormal eye movements. These signs warrant prompt medical care — the management is decided by your clinician.

Do I need to fast for a vitamin B1 test?

Usually no, but follow your lab's instructions (the draw may be bundled with other fasting tests). The tube is often shielded from light and sent to the lab quickly.

Vitamin B1 deficiency and pregnancy?

Severe pregnancy vomiting (hyperemesis gravidarum) can cause thiamine deficiency, sometimes Wernicke encephalopathy. If you have significant vomiting, tell your obstetric clinician.

Why is thiamine given before sugar (glucose)?

Because an IV glucose load in a deficient person "uses up" what little thiamine is left and can trigger Wernicke encephalopathy. So the medical rule is thiamine first. This is a clinician's decision in a care setting.

Should I take vitamin B1 supplements "for energy"?

Without a deficiency, no: excess of this water-soluble vitamin is cleared in the urine, and "anti-fatigue" supplementation has no proven benefit. Supplementation is useful in high-risk situations (alcohol, surgery, malnutrition), on medical advice.

Does low vitamin B1 mean cancer?

No. Thiamine is not a tumor marker. A low level mostly reflects your nutrition and body stores, not a tumor.

Bottom line

Vitamin B1 (thiamine) is a water-soluble vitamin essential for turning carbohydrates into energy, with small reserves (2–3 weeks). A deficiency — especially with heavy alcohol use, malnutrition, bariatric surgery, or vomiting — causes beriberi and Wernicke encephalopathy, which can leave lasting memory loss. Keep the ballpark in mind (~70–180 nmol/L in whole blood, lab-dependent), and remember that the diagnosis is clinical, that thiamine is given before glucose in an at-risk patient, and that supplementing "for fatigue" without a deficiency has no proven benefit. It is not a cancer marker. No value is read alone — it's your whole set of vitamins and your context that counts, which is what AI DiagMe provides, alongside your physician.

Sources

Official U.S. sources and peer-reviewed publications (PubMed) used for this guide:

Chandrakumar A, Bhardwaj A, 't Jong GW. Review of thiamine deficiency disorders: Wernicke encephalopathy and Korsakoff psychosis. J Basic Clin Physiol Pharmacol, 2019. PubMed · DOI ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷
NIH Office of Dietary Supplements — Thiamin — Fact Sheet for Health Professionals. ods.od.nih.gov ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹
Cantu-Weinstein A, Branning R, Alamir M, et al. Diagnosis and treatment of Wernicke's encephalopathy: A systematic literature review. Gen Hosp Psychiatry, 2024. PubMed · DOI ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷
Wijnia JW. A Clinician's View of Wernicke-Korsakoff Syndrome. J Clin Med, 2022. PubMed · DOI ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸
Panda A, Heidari A, Borumand M, et al. Thiamine deficiency in diabetes, obesity and bariatric surgery: Recipes for diabetic ketoacidosis. J Family Med Prim Care, 2024. PubMed · DOI ↩ ↩² ↩³ ↩⁴ ↩⁵
Bicer I, Dizdar OS, Dondurmacı E, et al. Furosemide-related thiamine deficiency in hospitalized hypervolemic patients with renal failure and heart failure. Nefrologia, 2023. PubMed · DOI ↩ ↩² ↩³ ↩⁴ ↩⁵
Nisar S, Mohi-U-Din K, Tak SI, et al. Thiamine responsive high output heart failure of adults: an under-recognized entity. Eur J Clin Nutr, 2023. PubMed · DOI ↩ ↩² ↩³ ↩⁴
Testing.com — Vitamin B1 (Thiamine) Test. testing.com ↩
MedlinePlus (U.S. National Library of Medicine, NIH) — Beriberi / Thiamine deficiency. medlineplus.gov ↩
Cleveland Clinic — Thiamine (Vitamin B1) Deficiency. my.clevelandclinic.org ↩
Sevransky JE, Rothman RE, Hager DN, et al. Effect of Vitamin C, Thiamine, and Hydrocortisone on Ventilator- and Vasopressor-Free Days in Patients With Sepsis (VICTAS). JAMA, 2021. PubMed · DOI ↩