I have the same thing with low magnesium inside the cell. I thought I remembered reading that thyroid hormone is responsible for getting magnesium inside the cell, and that calcium inside the cell is responsible for cell death. (Ray Peat) I thought I would google magnesium + carbon dioxide, because Ray Peat also frequently talked about the value of carbon dioxide. I found an article by Danny Roddy, who rewrites and retwists Ray Peat information. I was surprised to find him writing about magnesium because when I used to look at his articles he was recommending excessive amounts of calcium. Anyway, I can't take thyroid hormone, I think any hypothyroid I have is because of toxicity, and so the hormone only makes it worse. But maybe taking extra magnesium, or eating more foods containing magnesium, might help. I don't know. As for the carbon dioxide, coca cola makes me feel better; I buy the kind sweetened with real sugar. Glucose makes me feel better, too. I don't know if that helps get the magnesium (and potassium) inside the cell, too???? Link:
http://www.dannyroddy.com/main/2013/13/3/is-supplemental-magnesium-a-surrogate-for-thyroid-hormone
The reason I asked about the low BP and dehydration is that at times I have the same situation. There is an antidiuretic hormone called vasopressin, and it's antagonist, aldosterone, that affect sodium and potassium levels. I think I have always been deficient for some reason in vasopressin. Started peeing constantly since I was very young. Anyway, one of these, maybe the aldosterone, in some way can cause excessive fatigue. As usual, I have to say I don't know a whole lot, but maybe you would be interested in looking that up.