How can I differentiate between SIADH and other causes of hyponatremia in my patients?

Differentiating between Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) and other causes of hyponatremia primarily involves a careful assessment of the patient's fluid status and specific laboratory tests ¹, (Peri, 2019).

Key Differentiating Factors for SIADH:

• Fluid Status: Patients with SIADH are typically euvolemic, meaning they do not show clinical signs of fluid overload (e.g., oedema) or dehydration (e.g., reduced skin turgor, orthostatic hypotension) ¹, (Peri, 2019), (Warren et al., 2023). This is a crucial distinction from hypovolemic or hypervolemic hyponatremia ¹.
• Plasma Osmolality: SIADH is characterised by a low plasma osmolality, typically less than 275 mOsm/kg ¹, (Peri, 2019).
• Urine Osmolality: Despite low plasma osmolality, the urine osmolality in SIADH is inappropriately high, usually greater than 100 mOsm/kg ¹, (Peri, 2019), (Warren et al., 2023). This indicates the kidneys are concentrating urine excessively due to ADH ¹.
• Urine Sodium Concentration: Urine sodium concentration is typically high, often greater than 30 mmol/L, reflecting the kidney's inability to conserve sodium despite the hyponatremia ¹, (Peri, 2019), (Warren et al., 2023).
• Exclusion of Other Causes: A diagnosis of SIADH requires the exclusion of other conditions that can cause hyponatremia ¹, (Peri, 2019). This includes ruling out adrenal insufficiency, hypothyroidism, significant renal impairment, and recent diuretic use ¹, (Peri, 2019). Normal renal, adrenal, and thyroid function are expected in SIADH ¹.

Differentiation from Other Common Causes of Hyponatremia:

• Hypovolemic Hyponatremia: Patients present with signs of dehydration, such as dry mucous membranes, reduced skin turgor, or orthostatic hypotension ¹. The urine sodium concentration is typically low (less than 30 mmol/L) as the kidneys attempt to conserve sodium in response to volume depletion ¹. Causes include gastrointestinal losses (e.g., vomiting, diarrhoea) or excessive sweating ¹.
• Hypervolemic Hyponatremia: Patients exhibit signs of fluid overload, such as peripheral oedema, ascites, or pulmonary oedema ¹. This is commonly seen in conditions like heart failure, cirrhosis, or nephrotic syndrome ¹.
• Pseudohyponatremia: This occurs when severe hyperlipidaemia or hyperproteinaemia interfere with laboratory measurements, leading to a falsely low sodium reading ¹. In these cases, the plasma osmolality will be normal, which helps differentiate it from true hyponatremia ¹.
• Hyponatremia due to Hyperglycemia: High blood glucose levels can cause water to shift from cells into the extracellular space, diluting serum sodium ¹. In this scenario, the plasma osmolality will be elevated ¹.

Therefore, a comprehensive assessment including clinical fluid status and specific laboratory parameters (serum sodium, plasma osmolality, urine osmolality, and urine sodium) is essential for accurate differentiation ¹, (Peri, 2019).