What is magnesium serum?

Magnesium: I'm not sure what you mean. Assuming this means you were told by your doctor you had a abnormally high serum magnesium level, it can be caused by a number of things, some a simple as use of certain magnesium-containing antacids. It is also associated with kidney disease and if pathologic often associated with abnormal calcium and/or potassium levels. It definitely deserves further evaluation.