I know, for example, that blood levels of magnesium are not a very good indicator of magnesium levels in cells.
Are blood tests for sodium / potassium equally as unhelpful unless you are in dire straights?
I'm trying to hack muscle cramping (feet, sometimes hands) and tension headaches (contracting of scalp muscles around the head). When my doctor said my blood tests were normal (including sodium, potassium, magnesium and calcium), does that really rule out those electrolytes being the cause?
Serum electrolytes are fleeting measurements that exist within a very small tolerance range. High potassium may be just a fluke. Get tested again to be sure. Serum measurements do not measure intracellularly rather they provide an overall snapshot of fluid/electrolyte balance. If you know the levels in the blood, you can almost always deduce which direction things are shifting.. but not quite x mEq/L in cell A or cell B... what exactly are you trying to figure out?
electrolytes are also only as good as the fluid they are dissolved in, i.e.. h20. so many things to take into consideration and without seeing you in person alongside your labs, it's so hard to tell! keep working at it my friend