Hypermagnesemia refers to an abnormally high level of magnesium in the blood. The most common cause is kidney failure, but it can also be due to excessive magnesium intake, hypothyroidism (underactive thyroid gland), Addison’s disease, or it may be linked to hypercalcemia (high levels of calcium). Symptoms of hypermagnesemia may include lethargy, weakness, and irregularities in the heart’s electrical activity. As magnesium levels rise, it can even lead to a coma. The treatment for hypermagnesemia focuses on addressing the underlying cause.