Hyponatremia due to excessive water intake or syndrome of inappropriate antidiuretic hormone secretion (SIADH): This condition is characterized by an excessive release of antidiuretic hormone (ADH), leading to water retention and dilutional hyponatremia. It's a common cause of low sodium levels, especially in hospitalized patients or those with certain types of cancer.

Dehydration with secondary hyperaldosteronism: Although dehydration typically leads to hypernatremia, certain cases where there's a significant loss of sodium (e.g., through vomiting or diarrhea) can result in hyponatremia. The body's response to dehydration includes the release of aldosterone, which can further complicate sodium levels.
Heart failure: In heart failure, the decreased effective circulating volume triggers the release of ADH and aldosterone, leading to water and sodium retention. However, the sodium levels can become diluted due to increased water retention, resulting in hyponatremia.
Nephrotic syndrome: This condition involves significant proteinuria, leading to hypoalbuminemia. The decreased oncotic pressure can cause edema and potentially lead to hyponatremia due to dilution.

Adrenal insufficiency: This is a life-threatening condition where the adrenal glands do not produce adequate amounts of cortisol and aldosterone. Aldosterone deficiency can lead to significant sodium loss, resulting in hyponatremia. Missing this diagnosis can be fatal.
Diabetic ketoacidosis (DKA): Although DKA typically presents with hypernatremia due to glucose-induced osmotic diuresis, certain cases can present with hyponatremia, especially if the patient has been drinking large amounts of water or has experienced significant sodium loss.

Pituitary or hypothalamic disorders: Certain rare conditions affecting the pituitary or hypothalamus can disrupt ADH regulation, leading to SIADH or central diabetes insipidus, which can cause hyponatremia.
Congenital adrenal hyperplasia: This group of inherited disorders affects the adrenal glands, potentially leading to imbalances in cortisol, aldosterone, and sex hormone production. Some forms can result in hyponatremia due to aldosterone deficiency.
Cerebral salt wasting: A rare condition typically seen in patients with cerebral disorders, characterized by excessive renal sodium excretion, leading to hyponatremia and volume depletion.