Thiamin, also called vitamin B1, is an essential vitamin that is required to convert foodstuffs into energy and for the health and proper functioning of the nervous, muscular and cardiovascular systems.
In the bloodstream, 90% of active thiamin (TPP) is carried by red blood cells while 10% is transported in the bloodstream as free thiamin and thiamin monophosphate bound mostly to the protein albumin.
In the diet, animal food sources provide active thiamin while plant food sources provide free thiamin.1 
Urinary excretion of thiamin cannot be detected when vitamin intake is below the required levels. On the other hand, when intake exceeds saturation in the body, thiamin and/or its metabolites are actively excreted into urine to prevent excessive toxicity of the vitamins.2 
In patients who have thiamin deficiency, the most common conditions that bring them to a clinician include neuropathies, depression, myalgia, cardiomyopathies or takes diuretics and/or eat a high carbohydrate diet.3 
What Is Thiamin Deficiency In Celiac Disease and/or Gluten Sensitivity?
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Kathleen Mahan and Sylvia Escott-Stump, ed. Krause’s Food, Nutrition & Diet Therapy, 10th Edition. Philadelphia, PA. USA: W.B. Saunders Company, 2000. ↩ 
Shibata K, Hirose J, Fukuwatari T. Relationship Between Urinary Concentrations of Nine Water-soluble Vitamins and their Vitamin Intakes in Japanese Adult Males. Nutr Metab Insights. 2014 Aug 5;7:61-75. doi: 10.4137/NMI.S17245. ↩ 
Spectracell Labs, Inc. ↩