What are the causes of euvolemic hyponatremia (low sodium levels in the blood with normal fluid volume)?

From the Guidelines

Euvolemic hyponatremia is primarily caused by the Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH), hypothyroidism, adrenal insufficiency, and certain medications, which lead to an imbalance in water and sodium levels in the body. The causes of euvolemic hyponatremia can be broken down into several key categories, including:

• Hormonal imbalances, such as SIADH, where excessive release of ADH despite normal plasma osmolality leads to water retention 1
• Endocrine disorders, including hypothyroidism, which reduces cardiac output and decreases glomerular filtration rate, resulting in water retention
• Adrenal insufficiency, where cortisol deficiency increases ADH secretion
• Medication-induced hyponatremia, caused by certain antidepressants (SSRIs, TCAs), antipsychotics, anticonvulsants (carbamazepine, oxcarbazepine), and certain chemotherapeutic agents
• Other conditions, such as primary polydipsia, reset osmostat syndrome, and beer potomania syndrome, which can also contribute to euvolemic hyponatremia It is essential to identify the specific cause of euvolemic hyponatremia to provide appropriate management, which may include fluid restriction, medication adjustment, hormone replacement, or addressing the underlying condition, as treatment with vaptans, such as tolvaptan, has shown efficacy in improving serum sodium concentration in conditions associated with high vasopressin levels, such as SIADH 1.

From the Research

Causes of Euvolemic Hyponatremia

• The most common cause of euvolemic hyponatremia is the syndrome of inappropriate antidiuretic hormone secretion (SIADH) 2, 3, 4, 5.
• SIADH can be induced by various factors, including:
  • Tumors 2
  • Pulmonary diseases 2
  • Central nervous system disorders 2
  • Certain drugs, such as antidepressants and antiepileptics 2
• Other causes of euvolemic hyponatremia include:
  • Hypothyroidism 6
  • Adrenal insufficiency 6
  • Central hypothyroidism and central hypoadrenalism 6
• Clinical mimics of SIADH, such as cerebral salt wasting and reset osmostat, should also be considered in some patients 5.

Diagnosis and Treatment

• Diagnosis of SIADH involves evaluation of clinical and laboratory criteria, including decreased serum osmolality, inappropriately elevated urine osmolality, and elevated urine sodium levels 5.
• Treatment options for euvolemic hyponatremia include:
  • Fluid restriction 2, 3, 5
  • Administration of hyperosmolar saline solution in case of severe symptoms 2, 3
  • Therapy with tolvaptan, a selective vasopressin-V2-receptor antagonist 2
  • Loop diuretics, demeclocyclin, urea, and lithium 3
  • Free water restriction combined with increased solute intake (e.g., urea) 5
• It is essential to distinguish between acute and chronic hyponatremia, as well as to consider the clinical symptomatology, to initiate proper therapy 5.