From the Guidelines
The initial workup for hyponatremia of unknown etiology should include a thorough history, physical examination, and targeted laboratory tests, prioritizing the assessment of volume status and measurement of serum sodium, serum osmolality, urine sodium, and urine osmolality 1. The goal is to differentiate between hypovolemia, euvolemia, and hypervolemia, and to identify potential underlying causes such as SIADH, adrenal insufficiency, hypothyroidism, medication effects, and volume depletion or overload states. Key laboratory tests include:
- Thyroid function tests
- Morning cortisol
- Kidney function tests (BUN, creatinine)
- Liver function tests
- Glucose
- Calcium
- Potassium
- Medication review A detailed history of fluid intake, recent illnesses, and symptoms like nausea, confusion, or seizures is also crucial, as the severity of symptoms should guide management urgency 1. Understanding the underlying pathophysiology is essential, as hyponatremia represents a disorder of water balance rather than sodium balance in most cases, and the pattern of laboratory results will guide the correct diagnosis and appropriate treatment strategy 1. It is also important to note that the management of hyponatremia should be tailored to the individual patient, taking into account the severity of symptoms, the underlying cause, and the risk of complications such as central pontine myelinolysis 1.
From the Research
Initial Workup for Hyponatremia
The initial workup for hyponatremia should involve a systematic approach to laboratory diagnosis, including:
- Measurement of serum osmolality, urine sodium concentration, and urine osmolality to help differentiate among possible causes 2, 3
- Assessment of extracellular volume status on physical examination to provide clues as to the pathogenesis of hyponatremia 3, 4
- Estimation of serum sodium, urine electrolytes, and serum and urine osmolality, in addition to other case-specific laboratory parameters 4
- Point-of-care ultrasonography as an adjunct to physical assessment in estimation of volume status 4
Laboratory Evaluation
The laboratory evaluation of hyponatremia should include:
- Measurement of the effective serum tonicity (serum osmolality less serum urea level) as the first step in the laboratory evaluation 3
- Determination of urine osmolality to determine whether water excretion is normal or impaired 3
- Measurement of urine sodium level to differentiate between hypovolemia and the syndrome of inappropriate antidiuretic hormone secretion 3
- Determination of hormone levels (thyroid-stimulating hormone and cortisol) and arterial blood gases in difficult cases of hyponatremia 3
Clinical Assessment
The clinical assessment of hyponatremia should involve:
- Evaluation of the patient's fluid volume status, categorizing them as hypovolemic, euvolemic, or hypervolemic hyponatremia 5, 4
- Identification of the underlying cause of hyponatremia, such as medication use, diseases associated with hypervolemic states, or the syndrome of inappropriate antidiuretic hormone secretion 5, 2, 6
- Assessment of symptom severity, as mild chronic hyponatremia can be associated with cognitive impairment, gait disturbances, and increased rates of falls and fractures, while severely symptomatic hyponatremia can be life-threatening 5, 6