Yes, same with magnesium, the serum tests will never reveal the real picture, as 98 % is within the cells.
The thyroid tests frequently (or always) show a faulthy picture too. Well covered at this forum.
In general, dr. tests are made for medication..
A suggestion is to stabilize the body with sodium, potassium and magnesium.
Then toss in a good amount of vit C. Perhaps some vit B too, as the adrenals depend on it.
Then iodine/selenium.