Renal Failure: The kidneys are primarily responsible for magnesium excretion. Impaired renal function can lead to elevated magnesium levels as the body retains more magnesium than it should. Given the critical role of the kidneys in magnesium homeostasis, renal failure is a common and likely cause of hypermagnesemia.

Excessive Magnesium Intake: Consuming too much magnesium through supplements, antacids, or laxatives can lead to elevated magnesium levels, especially in individuals with normal renal function who might not be able to efficiently excrete the excess.
Hypothyroidism: This condition can lead to decreased renal function and thus impaired magnesium excretion, contributing to hypermagnesemia.
Adrenal Insufficiency: Similar to hypothyroidism, adrenal insufficiency can affect renal function and lead to elevated magnesium levels.

Rhabdomyolysis: Although less common, rhabdomyolysis (the breakdown of muscle tissue) can release magnesium into the bloodstream, leading to hypermagnesemia. Missing this diagnosis could be catastrophic due to the potential for severe kidney damage and other complications.
Tumor Lysis Syndrome: This condition, often associated with cancer treatment, involves the rapid release of cellular contents, including magnesium, into the bloodstream. It is a medical emergency and must not be overlooked.

Familial Hypocalciuric Hypercalcemia (FHH): A rare genetic disorder that can affect magnesium levels, though it is more commonly associated with alterations in calcium homeostasis.
Lithium Therapy: Long-term use of lithium can impair renal function, potentially leading to elevated magnesium levels in some cases.
Magnesium-Containing Enemas or Cathartics: Though less common with modern medical practices, the use of magnesium-containing products for bowel preparation can lead to absorption and subsequent hypermagnesemia, especially in patients with compromised renal function.