Hypovolemic Hyponatremia: This is often the most common cause of hyponatremia, resulting from a decrease in extracellular fluid volume, which can be due to various reasons such as diarrhea, vomiting, or excessive diuretic use. The body's response to volume depletion leads to the retention of water, diluting sodium levels.

Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): A condition where there is an excessive release of antidiuretic hormone (ADH), leading to water retention and subsequent dilutional hyponatremia. Common causes include pulmonary diseases, neurological disorders, and certain medications.
Heart Failure: Reduced cardiac output leads to decreased renal perfusion, triggering the renin-angiotensin-aldosterone system and resulting in water and sodium retention. However, the sodium retention is less than the water retention, leading to hyponatremia.
Nephrotic Syndrome: Characterized by heavy proteinuria, hypoalbuminemia, and edema. The hypoalbuminemia leads to a decrease in oncotic pressure, causing a shift of fluid from the vascular space to the interstitial space, which can result in hyponatremia due to dilution.

Adrenal Insufficiency: A life-threatening condition that can cause hyponatremia due to the lack of aldosterone, leading to an inability to retain sodium. It's crucial to identify and treat promptly.
Myxedema Coma (Severe Hypothyroidism): Although rare, it's a critical condition that can present with hyponatremia among other symptoms like altered mental status and hypothermia. Early recognition and treatment are vital.
Water Intoxication: Can occur due to excessive water intake, especially in individuals with psychiatric disorders or during endurance sports events. It leads to acute hyponatremia, which can be life-threatening if not recognized and managed promptly.

Pseudohyponatremia: A laboratory artifact seen in conditions with high lipid or protein levels, which can falsely lower the measured sodium concentration.
Reset Osmostat: A rare condition where the body's osmoregulatory mechanism is reset to maintain a lower osmolality, leading to hyponatremia.
Cerebral Salt Wasting: A condition associated with cerebral disorders, leading to excessive renal sodium loss and resulting hyponatremia. It's distinguished from SIADH by volume depletion.