Vitamin B1 (Thiamine) (B1)

The reference band above applies to whole-blood thiamine, which is the specimen that actually tracks stores. Some labs still report serum or plasma instead, and that distinction matters more here than for most vitamins.

The plainest version of this test measures thiamine floating in serum or plasma. The problem is anatomical: less than 10% of the thiamine in blood is contained in plasma. The other 90-plus percent is the active form, thiamine diphosphate, sitting inside red blood cells where the body uses it. Measuring only the free plasma fraction is like checking the change in someone's pocket to judge their bank balance.

That is why a normal serum thiamine and lingering symptoms can coexist, and why people search "normal thiamine but tired" after a clean-looking result. The fix is a better specimen. Whole blood is the preferred sample because thiamine diphosphate, roughly 80% of it localized in red cells, is a reliable indicator of total body stores. The ADLM testing guide describes LC-MS/MS measurement of whole blood or erythrocytes as the most sensitive and specific method available.

When even a whole-blood level looks ambiguous, a functional test sidesteps the question. Erythrocyte transketolase is a thiamine-dependent enzyme; the lab measures its activity, adds thiamine pyrophosphate, and measures again. If the enzyme jumps when fed thiamine, the cells were starving for it. The activity coefficient quantifies the jump: 1.0 to 1.15 is normal, 1.15 to 1.25 indicates moderate risk, and a coefficient above 1.25, equivalent to more than 25% stimulation by added thiamine pyrophosphate, indicates deficiency. A coefficient of 1.2, or an 18% stimulation, lands in that moderate-risk middle. The same review names erythrocyte transketolase activity as the best functional test and notes plainly that serum and plasma thiamine have low sensitivity and specificity.

Early thiamine deficiency is quiet and easy to blame on something else: fatigue, irritability, poor memory and brain fog, and loss of appetite. As it deepens it moves into the nerves, showing up as pins and needles in the toes, burning feet at night, and numb hands and weak legs, the picture of peripheral neuropathy described in clinical reference texts.

Left to run, it becomes beriberi. The "wet" form hits the heart and circulation, with legs swelling, breathlessness, and a fast heartbeat. The "dry" form is neurological, with numbness, weakness, and difficulty walking. The most feared endpoint is the brain. Severe deficiency can cause Wernicke disease and, if it progresses, Korsakoff syndrome. In industrialized countries, the NIH notes that deficiency is mainly associated with chronic alcohol use disorder, which both reduces intake and impairs how thiamine is absorbed.

One piece of timing turns this from a slow concern into an urgent one. Burning glucose is itself a thiamine-dependent process. Giving glucose or dextrose to someone whose stores are already empty can drive the last circulating thiamine into cells and precipitate or worsen Wernicke encephalopathy. The rule that follows is well established: thiamine should be given with or before glucose. It is why emergency departments reach for thiamine first.

Dietary targets give a sense of how little it takes to stay topped up day to day. The NIH sets the recommended dietary allowance at 1.2 mg per day for adult men and 1.1 mg per day for adult women. As a rough check on intake, the NIH also notes that urinary thiamin excretion below 100 mcg per day suggests insufficient intake, and below 40 mcg per day indicates extremely low intake. These are reference figures, not a treatment plan; the loading needed to correct an established deficiency is a separate clinical decision.

Thiamine rarely tells its whole story alone. It is usually drawn as part of a broader vitamins and nutrients panel, where it sits alongside relatives that share its B-vitamin neighborhood and overlap in symptoms. Neuropathy and fatigue also belong to shortfalls in vitamin B6, vitamin B12, and folate, so one low number is rarely read in isolation. Magnesium is worth a glance too, since thiamine's active form depends on it. For help putting the whole set together, the guide on reading a vitamin panel walks through how these results interact.

One more habit pays off with this marker specifically. Because the reserve is so short, a single snapshot can be misleading in either direction, caught on a good week or a bad one. A vitamin that can empty in weeks also rewards a tighter recheck rhythm than slower nutrients, so how often to repeat the test is worth settling with your doctor, especially during recovery or while a driver like heavy drinking is being addressed.