What is a common medication that can cause euvolemic hyponatremia?

Euvolemic hyponatremia is a condition characterized by low sodium levels in the blood while maintaining a normal fluid volume. Desmopressin is a synthetic analog of vasopressin (antidiuretic hormone) that acts on the kidneys to help retain water. In certain clinical situations, such as in patients with diabetes insipidus, the administration of desmopressin can lead to water retention without a significant change in the overall volume of extracellular fluid. This increased water retention can dilute serum sodium levels, resulting in euvolemic hyponatremia.

The other medications listed do not typically cause euvolemic hyponatremia as a common side effect. Instead, they are associated with different side effects or conditions unrelated to sodium balance in the context of euvolemia. For example, levothyroxine primarily affects thyroid hormone levels; amiodarone is an antiarrhythmic drug that can have multiple effects on the cardiovascular system; and metformin is a medication used for managing type 2 diabetes, which does not directly influence sodium levels. Therefore, desmopressin stands out as the clear medication associated with causing euvolemic hyponatremia.