A thiamine (vitamin B1) blood test measures the level of this essential, water-soluble vitamin. Thiamine acts as a vital cofactor in carbohydrate and amino acid metabolism and is necessary for the production of energy (ATP) within cells. It also plays a crucial role in nerve function and muscle contraction, including the heart muscle. It is used to diagnose thiamine deficiency, which can cause conditions such as beriberi and Wernicke-Korsakoff syndrome.