What is the etiology of hyponatremia (low sodium levels)?

From the Guidelines

Hyponatremia is a complex condition with multiple etiologies, including excessive water intake, medications, SIADH, hypovolemic and hypervolemic states, and endocrine disorders, and its management depends on the underlying cause, severity, and chronicity of the sodium imbalance. The most common causes of hyponatremia include:

• Excessive water intake relative to sodium, which can occur with psychogenic polydipsia or iatrogenic causes like hypotonic IV fluids
• Medications frequently implicated, such as thiazide diuretics, SSRIs, carbamazepine, and NSAIDs
• Syndrome of inappropriate antidiuretic hormone secretion (SIADH), resulting from conditions like malignancies, CNS disorders, pulmonary diseases, or certain medications
• Hypovolemic hyponatremia, which occurs with sodium and water losses where water is replaced but sodium isn't, as seen in vomiting, diarrhea, excessive sweating, or renal losses from diuretics
• Hypervolemic hyponatremia, which develops in conditions with impaired water excretion like heart failure, cirrhosis, and nephrotic syndrome
• Endocrine disorders, such as adrenal insufficiency, hypothyroidism, and diabetes, which can also cause hyponatremia 1. Recognizing the specific etiology is crucial for appropriate management, as treatment approaches differ significantly based on the underlying cause, severity, and chronicity of the sodium imbalance. The treatment of hyponatremia in cirrhotic ascites depends on etiology, chronicity, severity, and urgency, and may include fluid restriction, reduction or discontinuation of diuretics and laxatives, administration of hyperoncotic albumin, and/or vasopressin receptor antagonists (“vaptans”) 1. The management of hyponatremia should prioritize the correction of the underlying cause, and the use of vaptans, such as tolvaptan, has been shown to be effective in improving serum sodium concentration in patients with hypervolemic hyponatremia 1.

From the Research

Etiology of Hyponatremia

Hyponatremia is a complex condition with multiple etiologies, including:

• Water retention, which is the most common cause 2
• Defective water excretion, usually due to elevated serum vasopressin levels 3
• Excessive fluid intake 3
• Thiazide use, which can lead to hyponatremia in susceptible individuals, particularly the elderly 4, 5, 6
• Cation (sodium and potassium) depletion 4
• Osmotic inactivation of sodium 4
• Reduced ability to excrete free water 4

Risk Factors for Hyponatremia

Certain risk factors increase the likelihood of developing hyponatremia, including:

• Age, with the elderly being more susceptible 4, 5
• Female sex 4
• Low body mass 4
• Genetic susceptibility 4
• Comorbidities, such as heart failure, liver disease, or malignancy 5
• Use of certain medications, such as non-steroidal anti-inflammatory drugs, selective serotonin re-uptake inhibitors, or tricyclic antidepressants 5

Pathophysiology of Hyponatremia

The pathophysiology of hyponatremia involves:

• Impaired diluting ability due to thiazide use 3, 6
• Stimulation of vasopressin release 6
• Reduction of glomerular filtration and enhanced proximal water reabsorption 6
• Direct effect on water flow in the collecting duct 6
• Water retention caused by impaired water excretion combined with cation depletion 6