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Vitamins & Minerals

Vitamin B1 (Thiamine)

Vitamin B1 is a vitamin your body needs to turn food into energy and keep the nervous system healthy. Serious shortfalls are uncommon, but they matter — most often with heavy alcohol use or poor nutrition. The blood test is only a guide: a low reading is taken seriously, but a normal one does not fully rule a shortfall out, so your GP weighs it against how you are feeling.

Targeted Testing
CategoryVitamins & Minerals
Reference rangeSee your report
Evidence○Targeted Testing
Last updated3 July 2026
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What is vitamin b1 (thiamine)?

Thiamine (vitamin B1) helps your body break down and release energy from food. It also keeps your nervous system healthy — nerves rely on it to transmit signals properly. Because thiamine is a water-soluble vitamin, your body does not store large reserves of it. You need regular intake from food (meat, wholegrains, legumes and fortified cereals are good sources) to maintain adequate levels. The blood test measures thiamine in whole blood or plasma. One important limitation: less than 10 per cent of the thiamine in your blood is in the plasma (the liquid part that most routine tests measure), so the test has low sensitivity and low specificity. This means a low reading does not always confirm deficiency, and a normal reading does not always rule it out. For this reason, thiamine deficiency is often diagnosed clinically — based on your symptoms, your history and the circumstances — as much as by the number alone.

Why is it tested?

A thiamine test is most often ordered when deficiency is suspected, particularly in people whose circumstances put them at higher risk. Heavy alcohol use is one of the most common reasons — alcohol reduces how much thiamine the gut absorbs, and people drinking heavily may also be eating very little. Other risk groups include people with conditions that affect gut absorption, those who have had weight-loss surgery, and people with very limited or restricted diets. In hospitals, thiamine is sometimes checked before starting fluids containing glucose in a person who is malnourished, because giving glucose without adequate thiamine can trigger a serious neurological emergency (Wernicke encephalopathy — described below).

Reference range (Australia)

Use the reference interval printed on your report — thiamine is usually measured in whole blood, and a shortfall is often judged clinically as well as by the level nmol/L

There is no separate male/female reference interval for blood thiamine. Because less than 10 per cent of thiamine circulates in plasma, the blood test is an imperfect reflection of the body's true stores; clinical context and symptoms are central to interpretation. Always use the interval on your own report.

Reference intervals vary between laboratories. Always use the range printed on your own report.

What your result means

If your level is low

A lower-than-range thiamine level may reflect insufficient recent dietary intake, impaired absorption, or an increased demand (for example, during illness). Whether this represents true deficiency depends on your history, your symptoms and the clinical picture. Severe thiamine deficiency causes a serious condition called Wernicke encephalopathy — a neurological emergency marked by confusion, problems with eye movement and poor balance. If not treated quickly, Wernicke encephalopathy can lead to permanent brain damage. This is why clinicians take a low thiamine result seriously in high-risk people, even if symptoms are not obvious yet.

If your level is in range

A thiamine result within the range on your report is generally reassuring. However, because the blood test is an imperfect measure — less than 10 per cent of body thiamine is in the plasma — your GP may still consider thiamine deficiency if your symptoms and history point that way.

If your level is high

A higher-than-range thiamine level is uncommon in routine testing and is not usually a clinical concern. It most often reflects recent supplementation or a thiamine-rich meal before the test. Excess thiamine from dietary sources is excreted by the kidneys.

What can affect your result

Diet is the most direct driver. Thiamine is found in meat and poultry, fish, wholegrains, legumes and fortified cereals. A very restricted or monotonous diet — or one relying heavily on heavily processed foods — can lead to a low intake over time. Alcohol reduces how well the gut absorbs thiamine and increases how fast the body uses it up. Chronic heavy alcohol use is one of the most common causes of thiamine deficiency. Gut absorption problems from conditions like coeliac disease or inflammatory bowel disease can lower thiamine levels, as can certain surgeries that alter the gut. The test itself matters too. Because most thiamine is inside red blood cells rather than in plasma, the sample type (whole blood versus plasma) significantly affects the result. A haemolysed sample — one where some red blood cells have broken open during collection — can also alter the reading. Your laboratory's reference interval is set for the method and sample type they use.

When to act

If your thiamine result is low and you are in a higher-risk group — heavy alcohol use, a very restricted diet, recent gut surgery, or recent rapid weight loss — that is worth a prompt conversation with your GP rather than waiting. If you have symptoms that could fit thiamine deficiency — confusion, problems with coordination, visual disturbances, tingling in hands or feet — your GP will weigh the blood level alongside those signs to decide on next steps. For most people with a mildly low or borderline result and no concerning symptoms, your GP will review your diet and circumstances and decide whether a repeat test or dietary changes make sense. This page is general information and is not a substitute for advice from your GP.

Frequently asked questions

Thiamine (vitamin B1) helps your body break down and release energy from food — it is involved in how carbohydrates are processed into the fuel your cells run on. It also keeps your nervous system healthy, supporting the signals that nerves send and receive. Because your body cannot make it, you need regular dietary intake from sources like meat, wholegrains, legumes and fortified cereals.

The highest-risk group is people with heavy alcohol use, which both reduces gut absorption and increases how quickly the body uses thiamine. Others at risk include people with conditions that impair gut absorption, those who have had weight-loss surgery, and people eating very restricted diets. Thiamine is an essential nutrient, so prolonged low intake eventually causes deficiency.

It is a useful starting point but has real limitations. Less than 10 per cent of thiamine in blood circulates in plasma, which is what most standard tests measure. This gives the test low sensitivity and low specificity — meaning a normal result does not fully rule out deficiency, and a low result is not always the whole story. Thiamine deficiency is often a clinical diagnosis: your GP weighs the blood level alongside your symptoms, history and circumstances.

Wernicke encephalopathy is a serious neurological emergency caused by a lack of thiamine. It can cause confusion, abnormal eye movements and problems with balance and coordination. If not treated quickly, it can lead to permanent brain damage. It most often occurs in people with heavy alcohol use or severe malnutrition. This is why thiamine deficiency is taken seriously even before obvious symptoms appear in high-risk people.

That is a question for your GP, based on your result, your symptoms and your circumstances. Most people who eat a varied diet get adequate thiamine from food. If a genuine deficiency is identified — particularly in a high-risk situation — your GP can advise on whether supplementation is appropriate and in what form. It is worth raising at your appointment rather than starting on your own.

Test your vitamin b1 (thiamine)

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