Abnormal levels of magnesium are most frequently seen in conditions or diseases that cause impaired or excessive excretion of magnesium by the kidneys or that cause impaired absorption in the intestines. Magnesium levels may be checked as part of an evaluation of the severity of kidney problems and/or of uncontrolled diabetes and may help in the diagnosis of gastrointestinal disorders.

Since a low magnesium level can, over time, cause persistently low calcium and potassium levels, it may be checked to help diagnose problems with calcium, potassium, phosphorus, and/or parathyroid hormone (involved with calcium regulation).

Magnesium levels may be measured frequently to monitor the response to oral or intravenous (IV) magnesium supplements and may be used, along with calcium and phosphorus testing, to monitor calcium supplementation.

Magnesium testing may be ordered as a follow-up to chronically low levels of calcium and potassium. It also may be ordered if you have symptoms of an abnormally low magnesium level such as muscle weakness, twitching, cramping, confusion, cardiac arrhythmias, and seizures.

Although dietary deficiencies of magnesium are rare, your doctor may order a magnesium level to check for a deficiency as part of an evaluation of malabsorption, malnutrition, diarrhea, or alcoholism. If you are taking certain medications that can cause the kidneys to excrete magnesium, testing may be performed as well. If magnesium and/or calcium supplementation is necessary, then the magnesium blood level most likely will be checked at intervals to monitor the effectiveness of treatment.

If you have a kidney disorder or uncontrolled diabetes, your doctor may order magnesium levels to help monitor kidney function and to make sure that you are not excreting or retaining excessive amounts of magnesium.

Low levels of magnesium (hypomagnesemia) in your blood may mean that you are: 1) not getting enough magnesium in your diet; 2) your intestines are not absorbing enough magnesium; or 3) your kidneys are excreting too much magnesium. Deficiencies may be due to: 

• Low dietary intake (seen in the elderly, malnourished, and with alcoholism) 
• Gastrointestinal disorders (such as Crohn’s disease) 
• Uncontrolled diabetes 
• Hypoparathyroidism (underactive parathyroid gland)
• Long-term diuretic use 
• Prolonged diarrhea 
• Post surgery 
• Severe burns 
• Toxemia of pregnancy

Increased levels of magnesium are rarely due to dietary sources but are usually the result of an excretion problem or excessive supplementation. Increased levels are seen in: 

• Kidney failure 
• Hyperparathyroidism (overactive parathyroid gland)
• Hypothyroidism 
• Dehydration 
• Diabetic acidosis (when first seen) 
• Addison’s disease
• Use of magnesium-containing antacids or laxatives

Since magnesium is an electrolyte, it may be ordered along with other electrolytes such as sodium, potassium, chloride, CO2, calcium, and phosphorus to evaluate a patient’s electrolyte balance. If magnesium is low, it is not unusual for potassium also to be low.

Magnesium blood levels may be low normally in the second and third trimesters of pregnancy.