Vitamin B1 (Thiamine)

What Vitamin B1 (Thiamine) actually measures — the analyte, and the assay

The thiamine that matters is not free thiamine, and it is not in your plasma. Inside cells the vitamin is phosphorylated to thiamine diphosphate, and that ester is the cofactor for four enzymes that between them run the entry to the citric acid cycle and the non-oxidative pentose phosphate pathway — pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, the branched-chain ketoacid dehydrogenase and transketolase. Roughly four fifths of the thiamine in blood is diphosphate sitting inside red cells.

That is why the sample is whole blood. The 70–180 nmol/L interval on this page belongs to a whole-blood thiamine diphosphate measured by high-performance liquid chromatography, usually after derivatization to the fluorescent thiochrome. A serum or plasma thiamine is a different analyte in a different matrix and the same interval does not apply to it.

The older test measures the enzyme instead of the vitamin. The erythrocyte transketolase activation coefficient runs the enzyme twice, with and without added cofactor, and reports how much the added cofactor accelerated it. A large activation means the enzyme was short. Direct erythrocyte thiamine diphosphate was formally evaluated against that functional assay in 1988 and has largely replaced it Baines 1988, but the two report on different scales — a percentage against a concentration — and neither converts into the other.

Plasma thiamine is not worthless, which is the correction this page owes its own framing. In rural Cambodian women followed through a thiamine fortification program, plasma thiamine and erythrocyte thiamine diphosphate showed comparable performance characteristics in response to the intervention McCann 2017. Plasma tracks recent supply and the red cell pool tracks the store; in a population whose supply is changing, both move.

The molecule is fragile. Thiamine esters degrade with light and with warmth, so a tube that sat on a bench in a bright room is reporting the room as well as the reader.

Vitamin B1 (Thiamine): what changes the blood, and what only changes the reading

What changes the thiamine in your body:

1. Alcohol, which cuts on three fronts at once: intake, intestinal absorption, and the phosphorylation step that converts thiamine into the active diphosphate.
2. Loop diuretics — acutely, and by a measured amount. In healthy volunteers given furosemide, urine flow rose from 51 ± 17 mL/h at baseline to as much as 183 ± 58 mL/h, and the thiamine excretion rate roughly doubled, from 6.4 ± 5.1 nmol/h to 11.6 ± 8.2 nmol/h (P < 0.01), correlating with urine flow at r = 0.54 Rieck 1999.
3. But the chronic version of that claim is weaker than it is usually stated. In a stable heart failure outpatient cohort on standard loop diuretic therapy, estimated deficiency prevalence was under 11.6%, and there was no correlation between diuretic dose and thiamine level (r = 0.02, P = 0.93) Teigen 2016. Spironolactone has been examined separately in heart failure Rocha 2008. Acute renal loss is demonstrated; a dose-response in chronic use is not.
4. Carbohydrate load. Every one of the four thiamine-dependent enzymes sits on a carbohydrate-handling pathway, so a large carbohydrate intake — oral or intravenous — raises cofactor demand rather than supply.
5. Magnesium status. The thiamine diphosphate-dependent enzymes need magnesium at the active site, which is why a repletion attempt can stall in someone magnesium-depleted and why Magnesium RBC is this marker's most useful neighbor.
6. Malabsorption after bariatric surgery, where intake looks adequate and delivery is not.

What changes only the reading:

1. Which matrix was drawn. Whole blood, washed erythrocytes, plasma and serum give four different numbers for one vitamin, and only one of them belongs to the interval printed beside your result.
2. Your hematocrit. Whole-blood thiamine diphosphate is a concentration across cells plus plasma, and the cofactor is almost entirely in the cells. Anemia dilutes it downward and a high hematocrit concentrates it upward without a molecule of thiamine changing hands. This is the single most overlooked artifact on the test.
3. A dose taken before the draw. Supplemental thiamine raises plasma within hours while the red cell pool lags, so a same-morning capsule flatters a plasma result and barely touches a whole-blood one.
4. Light and delay. Degradation between the needle and the freezer lowers the result.
5. Which test the requisition asked for. An activation coefficient reported as a percentage next to a 70–180 nmol/L interval is two tests printed in one column Baines 1988.

Reference interval or decision threshold — which kind of number Vitamin B1 (Thiamine) is

70–180 nmol/L is a reference interval, tied to a matrix and a method. It is the central band of a measured population using whole-blood thiamine diphosphate by chromatography. Move to plasma, or to a laboratory reporting per gram of hemoglobin, and the numbers on the page stop applying.

The lower limit is a flag, not a finding. A value under 70 nmol/L says the reader sits below where most of a reference population sat. It does not, on its own, describe a clinical state, and the site's own optimal line says the same thing in the other direction.

There is no established optimum above the floor, and this is one of the honest places on the site to say so plainly. Thiamine is water-soluble with a small body pool of roughly 25–30 mg; excess intake is largely excreted, and no cohort has shown a benefit to sitting at 175 nmol/L rather than 110.

The activation coefficient is a third kind of number again — neither an interval nor an outcome threshold, but a functional ratio in which a larger value means a hungrier enzyme Baines 1988.

How you would know your Vitamin B1 (Thiamine) was wrong — and when to redraw

The red cell sets the pace, and it is measured in weeks. Thiamine entering a mature erythrocyte is phosphorylated and held there, and the cell itself survives about 120 days, so the whole-blood pool re-equilibrates over roughly 8 to 12 weeks after intake changes. Anything faster than that is measuring plasma, whatever the tube said.

Conditions for a comparable second draw: same matrix and same laboratory; no supplemental dose within 24 hours; the sample protected from light and separated promptly; and, if possible, a hematocrit from the same sitting so the dilution effect can be seen rather than guessed.

What would have to change for the second value to mean something:

• If the first result was low and the second is normal after supplementation, that is expected and proves supply, not need. The informative version is the reverse: stop the supplement for 8 weeks and see whether it falls back, which distinguishes a low intake from a low absorption.
• Check Magnesium RBC before concluding that repletion failed. A thiamine level that will not rise in someone taking it reliably is a magnesium question as often as a thiamine one.
• Check the CBC. A whole-blood thiamine that moved by 15% while the hematocrit moved by 15% has not moved.
• If a clinical syndrome is the reason for testing, the level is not the arbiter. Blood thiamine can be unremarkable in someone with neurological features, and treatment decisions in that situation belong to a physician and not to a number on a portal.

What Vitamin B1 (Thiamine) cannot tell you

It cannot tell you what is happening in the brain. Thiamine transport into the central nervous system is carrier-mediated and separate from the blood pool, so a normal whole-blood diphosphate does not exclude a neurological deficiency state. This is the most consequential blind spot the test has.

It cannot tell you the enzyme is working. A concentration is not an activity. That is precisely what the transketolase activation coefficient was built to answer and why it has not entirely disappeared Baines 1988.

It cannot separate cause. Low intake, impaired absorption, renal loss and increased demand all arrive as the same low number Rieck 1999 Teigen 2016.

It cannot be used to titrate a dose. No target concentration has been shown to correspond to a clinical benefit, so there is nothing to titrate toward.

The wrong inference readers actually draw is that a result in the middle of the interval rules thiamine out. On a diuretic, with a high carbohydrate intake, with a hematocrit at the top of its range, a mid-interval whole-blood value is entirely compatible with a person whose demand exceeds their supply.

Sources read for these sections

• Baines M, et al. The evaluation of erythrocyte thiamin diphosphate as an indicator of thiamin status in man, and its comparison with erythrocyte transketolase activity measurements. Annals of Clinical Biochemistry 1988 · PMID 3254112
• McCann A, et al. Comparable Performance Characteristics of Plasma Thiamine and Erythrocyte Thiamine Diphosphate in Response to Thiamine Fortification in Rural Cambodian Women. Nutrients 2017 · PMID 28661435
• Teigen LM, et al. Prevalence of thiamine deficiency in a stable heart failure outpatient cohort on standard loop diuretic therapy. Clinical Nutrition 2016 · PMID 26923517
• Rieck J, et al. Urinary loss of thiamine is increased by low doses of furosemide in healthy volunteers. Journal of Laboratory and Clinical Medicine 1999 · PMID 10482308
• Rocha RM, et al. Influence of spironolactone therapy on thiamine blood levels in patients with heart failure. Arquivos Brasileiros de Cardiologia 2008 · PMID 18516403