It almost feels like refeeding syndrome like perhaps the metabolism was w switched on too quickly and my body is struggling to adapt to those changes . I have to be ridiculously vigilant about my electrolytes or they tank.
@Bobbi007 - 100 mg. of thiamine (oral) caused my phosphorous to tank badly. At first it gave me very nice energy for a day or so, and then severe fatigue hit. It's a long story and unfortunately it took me a couple of years to figure this out. So initially though it made me feel so good I had to stop it, and this happened a couple of times. Until I figured out it was due to refeeding syndrome and hypophosphatemia is the hallmark of refeeding syndrome. Potassium, btw, did nothing to help with this.
Research showed dairy is high in phosphorous so I drank probably 3 glasses of kefir (it was several years ago) and within a few hours the horrible fatigue abated. I eventually got a phosphorous supplement and I'm able to take thiamine with no problem now.
I also had a refeeding syndrome reaction to methylfolate - first I had great energy for a day or so, and then boom - the fatigue hit. I was aware of the possiblity of my potassium tanking so was prepared to add it in and I titrated up to 1000 mg a day of potassium gluconate (in divided doses) over a couple of days, until the fatigue abated, and have taken it ever since (2010).
So you might be having a refeeding syndrome reaction to the thiamine though 3 weeks does seem like a long time to still be having symptoms, but you never know. I would try some phosphorous. I don't drink kefir daily or have much in the way of dairy products but it's a good way to see if it helps your symptoms and then you can decide if you want or need to take a phosphorous supplement.
Low phosphorous caused severe fatigue and weakness, double vision and pills sticking in my throat. It affects the small muscles first (eye and throat) but will end up making everything weak.
Why did the doctor give you the thiamine by IV? I thought IVs were usually used to deliver larger doses of nutrients over a period of time. Thiamine tablets work great for me.
This article has a good definition and description of Refeeding Syndrome:
https://www.ncbi.nlm.nih.gov/pmc/ar... syndrome?,whether enterally or parenterally5).