[bitL]

No, unfortunately, the serum level of B1 is not a good indicator of deficiency in tissues. There are some proxy tests like measuring transketolase or pyruvate but only a few labs in the world do them and one typically needs high-performance liquid chromatography etc.

See e.g. here:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6392124/

That's why I think people should just try megadosing B1 for a while as it is not toxic and to get a datapoint if their troubles can be related to a deficiency somewhere. Many people get instant relief of their problems just after a single dose but the effect might get weaker as the supplementation continues. However, it's a good hint for their physician where to look at.