Vitamin B1 (thiamine)

A B vitamin your cells need to turn carbohydrate into energy. The body stores only a few weeks of it.

The nervous system and the heart feel a shortage first, and heavy drinking is the commonest reason for one.

Most labs measure it in whole blood rather than serum, since almost all of it sits inside red cells -- a serum result is a different and less useful measurement. It degrades in light, so handling matters.

Worth discussing a low value, particularly with numbness or unexplained breathlessness.