Thiamine has been measured in a couple of studies.
Naviaux found it was higher on average among in cfs patients in men but not women (nb log scale, levels are extremely uneven between people and a good chunk of the difference in averages is driven by outliers)
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Hanson 2018 found patients were higher too, although most people fell below the threshold of detetction which is why many of the results are the same. (note again log scale, note also colours reversed from above)
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Overall this is not strong data but it is certainly not clear evidence of widespread thiamine deficiency. It doesn't follow though that supplementation doesn't help. Could be there are other problems that mean having super high levels of thiamine is useful. I note that thiamine is tied up in the function of pyruvate dehydrogenase
, which is hypothesised to be blocked.