Differential Diagnosis for Polyuria and Hyponatremia

Single Most Likely Diagnosis

• Diabetes Insipidus (Central or Nephrogenic): This condition is characterized by the inability to regulate fluids in the body due to a problem with the antidiuretic hormone (ADH), leading to polyuria. Hyponatremia can occur if the patient is unable to adequately replace lost sodium.

Other Likely Diagnoses

• Primary Polydipsia: Excessive water intake can lead to polyuria and, if the water intake is extremely high, can dilute sodium levels in the blood, causing hyponatremia.
• Nephrotic Syndrome: This condition involves significant loss of protein in the urine, which can lead to hyponatremia due to hypoalbuminemia and subsequent fluid shifts. Polyuria can occur due to the loss of proteins that help regulate fluid balance.
• Heart Failure: Decreased cardiac output can lead to decreased renal perfusion, activating the renin-angiotensin-aldosterone system, which can cause water retention and dilutional hyponatremia. Polyuria is less common but can occur in the context of diuretic use.

Do Not Miss Diagnoses

• Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH): Although SIADH typically presents with euvolemia or mild volume expansion, it is crucial not to miss this diagnosis as it can lead to severe hyponatremia. Polyuria is not a typical presenting symptom but can occur after treatment with fluid restriction or vasopressin antagonists.
• Adrenal Insufficiency: This condition can lead to hyponatremia due to the lack of aldosterone, which regulates sodium balance. Polyuria can occur due to the lack of cortisol and aldosterone, affecting the body's ability to regulate fluids.
• Diuretic Abuse: The use of diuretics, especially in the context of eating disorders or factitious disorder, can lead to both polyuria and hyponatremia.

Rare Diagnoses

• Barter Syndrome and Gitelman Syndrome: These are rare genetic disorders affecting the kidneys' ability to reabsorb sodium and potassium, leading to hypokalemia, metabolic alkalosis, and often hyponatremia. Polyuria can be a feature due to the kidney's impaired ability to concentrate urine.
• Liddle Syndrome: A rare genetic disorder characterized by excessive sodium reabsorption in the distal nephron, leading to hypertension and hypokalemia. While less commonly associated with hyponatremia and polyuria, it's an important consideration in the differential diagnosis of disorders affecting renal sodium handling.