I said she has long covid, including POTS, and apart from the POTS all medical tests are coming up negative/normal.
You don't know what tests were done, you don't know any of her other symptoms. I don't understand how you can be "pretty sure she is thiamine deficient".
If you are right and she can fix a year of misery by taking an over-the-counter supplement then I will be over the moon, but in order to convince her to try yet another remedy-suggested-by-someone-on-the-internet I'd like to hear more about your diagnosis
I can only imagine as you did not communicate them. But let me guess: fatigue, brain fog, possibly panic attacks, tingling/burning sensations in feet or hands, limbs numbing especially at night, the sensation of suffocation or dissatisfaction with breathing, the sense of a blocked nose while there is no stuff in there, headaches, spatial disorientation, loss of short-term memory, impacted ability to speak and learn new things, GI and cardiovascular issues. All that while having a normal (>= 96%) O2 saturation in the blood.
>I don't understand how you can be "pretty sure she is thiamine deficient".
She may have an acquired mitochondrial dysfunction caused by the oxidative stress induced by the active phase of illness. Which is unfortunately pretty typical for Covid [1] and 30% of survivors will have metabolic and neurological complications as the result, regardless of the severity of the original illness.
This is the first thing to check given the symptoms, but most of neurologists are unaware of it yet. You can do a formal test that checks for that condition: transketolase.
I am also including some materials that might help: [2], [3]
You don't know what tests were done, you don't know any of her other symptoms. I don't understand how you can be "pretty sure she is thiamine deficient".
If you are right and she can fix a year of misery by taking an over-the-counter supplement then I will be over the moon, but in order to convince her to try yet another remedy-suggested-by-someone-on-the-internet I'd like to hear more about your diagnosis