@Mary Did you check if your low phosphate may be related to blood alkalosis (either respiratory alkalosis or metabolic alkalosis). Do you know your blood bicarbonate and breath rate?
@pattismith - no to all of that. I wouldn't even know how to check my blood bicarbonate though it might be interesting to know what it is. I believe my low phosphate was related to thiamine. I was "ok" (as okay as one can be with ME/CFS) before I started taking thiamine - at baseline. When I took thiamine, within a day I had a noticeable surge in energy, really nice surge. And then a day later was hit with severe fatigue. This happened to me twice. The first time I tried thiamine I had to stop it because although the energy surge was great, the following fatigue was too severe.
The second time I tried it, I figure out that the resulting fatigue after the energy surge was due to hypophosphatemia, due to refeeding syndrome, and once I supplemented with phosphate, the fatigue dissipated. So there was no need for me to investigate this further. I've been able to take thiamine ever since, and I still take a phosphate supplement about 2 x a week. It's very similar to what happened when I started taking methylfolate - nice increase in energy, followed by severe fatigue due to potassium tanking. I started taking potassium, and was able to keep taking methylfolate, and have had to take potassium ever since.