Magnesium level in blood increases in:
The concentration of magnesium in serum is closely regulated within the range of 0.7–1 mmol/L (1.5–2 meq/L; 1.7–2.4 mg/dL), of which 30% is protein-bound and another 15% is loosely complexed to phosphate and other anions. One-half of the 25 g (1000 mmol) of total body magnesium is located in bone, only one-half of which is insoluble in the mineral phase. Almost all extraskeletal magnesium is present within cells, where the total concentration is 5 mM, 95% of which is bound to proteins and other macromolecules. 
Because only 1% of body magnesium resides in the ECF, measurements of serum magnesium levels may not accurately reflect the level of total body magnesium stores.
Regulation of serum magnesium concentrations is achieved mainly by control of renal magnesium reabsorption. Only 20% of filtered magnesium is reabsorbed in the proximal tubule, whereas 60% is reclaimed in the cTAL and another 5–10% in the DCT. Magnesium reabsorption in the cTAL occurs via a  paracellular route that requires both a lumen-positive potential, created by NaCl reabsorption and tight-junction proteins encoded by members of the Claudin gene family. Magnesium reabsorption in the cTAL is increased by PTH, but inhibited by hypercalcemia or hypermagnesemia, both of which activate the CaSR in this nephron segment.
 
Reference: :Harrison’s medicine page no  2916
Uncontrolled DM
Liver cirrhosis
Kidney failure
Chronic alcoholism