Interpretation: Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)
This laboratory pattern—low serum osmolality (274 mOsm/kg), inappropriately elevated urine osmolality (363 mOsm/kg), and high urine sodium (81 mEq/L)—is diagnostic of SIADH. 1
Diagnostic Reasoning
The key to interpreting these values lies in understanding the relationship between serum and urine osmolality:
- Serum osmolality of 274 mOsm/kg is low (<275 mOsm/kg), indicating hyponatremia and a hypotonic state 1
- Urine osmolality of 363 mOsm/kg is inappropriately concentrated given the low serum osmolality—the kidneys should be producing dilute urine (<100 mOsm/kg) in response to low serum osmolality, but instead they are concentrating urine 1
- Urine sodium of 81 mEq/L is elevated (>30 mEq/L), confirming ongoing natriuresis despite hyponatremia 2
In SIADH, urine osmolality is inappropriately high (>300 mOsm/kg, often >500 mOsm/kg) despite low serum osmolality (<275 mOsm/kg). 1 This patient's values fit this pattern precisely.
Confirming SIADH Diagnosis
Before finalizing the diagnosis, verify the following exclusions:
- Rule out hyperglycemia and elevated urea, as these can falsely elevate calculated osmolality 3, 4
- Exclude volume depletion, adrenal insufficiency, hypothyroidism, heart failure, cirrhosis, and diuretic use—all of which can mimic SIADH 1
- Confirm euvolemic status clinically, as SIADH patients are typically euvolemic (not hypovolemic or hypervolemic) 1
Clinical Significance of High Urine Sodium
The urine sodium of 81 mEq/L is notably elevated and suggests that fluid restriction alone may be insufficient for treatment. 2 Research demonstrates that very high urine sodium concentrations (>130 mmol/L) in severe SIADH predict poor response to fluid restriction alone, though this patient's value of 81 mEq/L is moderately elevated 2.
Critical Management Pitfall
Avoid administering intravenous fluids or encouraging oral fluid intake, as this will worsen the hyponatremia. 2 In SIADH, the kidneys retain water inappropriately, so additional fluid administration paradoxically lowers serum sodium further by diluting the serum while the kidneys continue to excrete sodium 2.
Contrast with Other Conditions
This pattern clearly distinguishes SIADH from:
- Diabetes insipidus: Would show high serum osmolality (>300 mOsm/kg) with inappropriately low urine osmolality (<300 mOsm/kg) 4, 5
- Hypovolemic hyponatremia: Would typically show urine sodium <30 mEq/L as the kidneys attempt to conserve sodium 2
- Dehydration: Would show elevated serum osmolality (>300 mOsm/kg), not low 3, 4
Next Steps
- Initiate fluid restriction as first-line therapy 1
- Identify and treat the underlying cause of SIADH (malignancy, CNS disorders, pulmonary disease, medications) 2, 6
- Monitor serum sodium closely during treatment, as overly rapid correction can cause osmotic demyelination syndrome 1
- Consider additional therapies (hypertonic saline, vasopressin receptor antagonists, or urea) if fluid restriction fails, particularly given the moderately elevated urine sodium 2