From the Guidelines
For euvolemic hyponatremia, I recommend ordering serum osmolality, urine osmolality, urine sodium, thyroid function tests (TSH, free T4), morning cortisol, and serum uric acid. These labs are crucial in distinguishing between the Syndrome of Inappropriate Antidiuretic Hormone (SIADH) and other causes of euvolemic hyponatremia, as they help in assessing the osmoregulatory function and identifying potential underlying causes such as hypothyroidism or adrenal insufficiency 1.
Key Labs to Order
- Serum osmolality to confirm true hyponatremia, which is typically low (<280 mOsm/kg)
- Urine osmolality to assess the kidneys' ability to concentrate urine, often inappropriately high (>100 mOsm/kg) in SIADH
- Urine sodium to evaluate the kidneys' handling of sodium, with levels >30 mEq/L suggesting SIADH in the context of normal kidney function
- Thyroid function tests (TSH, free T4) and morning cortisol to rule out hypothyroidism and adrenal insufficiency, respectively
- Serum uric acid, as its level can be affected in various conditions leading to euvolemic hyponatremia
Additional Considerations
- A comprehensive medication history is essential, as several drugs (e.g., SSRIs, carbamazepine, thiazide diuretics) can cause SIADH
- In unclear cases, further tests such as the BUN/creatinine ratio and fractional excretion of uric acid may aid in confirming the diagnosis
- A comprehensive metabolic panel should also be considered to assess kidney function and identify other electrolyte abnormalities that may accompany hyponatremia, thus guiding the management and improving outcomes in terms of morbidity, mortality, and quality of life.
From the Research
Euvolemic Hyponatremia Diagnosis
To diagnose euvolemic hyponatremia, the following laboratory tests can be ordered:
- Serum sodium level
- Serum osmolality
- Urine osmolality
- Urine sodium level
- Thyroid function tests to rule out hypothyroidism 2
- Adrenal function tests to rule out adrenal insufficiency 2
- Basic metabolic panel to evaluate electrolyte balance and renal function 3
Laboratory Evaluation
The laboratory evaluation of euvolemic hyponatremia should include:
- Measurement of serum and urine osmolality to determine the presence of inappropriate antidiuretic hormone secretion (SIADH) 4
- Assessment of urine sodium level to evaluate the patient's volume status and renal function 4
- Evaluation of thyroid and adrenal function to rule out underlying endocrinopathies 2
- Point-of-care ultrasonography to estimate volume status 5
Diagnostic Criteria
The diagnostic criteria for SIADH, a common cause of euvolemic hyponatremia, include: