Magnesium in the blood stream is excreted by the kidney.

Without a "trigger" like diabetes, it is conserved to some extent.

I have not found any other information about serum levels that promote excretion. So other medical conditions may exist and not be researched yet.

But diuretics promote excretion to such an extent that people may die from that loss. Many on diuretic therapy for congestive heart failure who take high dose Lasix end up in ERs and some may die. When Propulsid was on the market there were deaths from prolonged QT events of the heart that occurred from low mag levels. This was the main reason it was removed.

There are many drugs on the QT lists, that can cause heart rhythm anomalies, when magnesium is low.

http://emedicine.medscape.com/article/246489-overview
This article explains hypermagnesemia.
The two major ways it can occur is thru IV supplements (used in treating Eclampsia of pregnancy this way, some IV chemo replacement), or rectal overuse of enemas containing magnesium like Fleets.