From the Guidelines
To investigate hyponatremia, a thorough history and physical examination focusing on volume status, medications, and symptoms should be obtained, followed by basic laboratory tests including serum sodium, potassium, chloride, bicarbonate, blood urea nitrogen, creatinine, glucose, and serum osmolality, as well as a urine sample for sodium, potassium, creatinine, and osmolality, as recommended by the most recent guidelines 1. The initial step in investigating hyponatremia involves assessing the patient's volume status by categorizing them as hypovolemic, euvolemic, or hypervolemic based on clinical findings and laboratory results.
- For hypovolemic hyponatremia, urine sodium levels should be checked, with levels typically being <20 mmol/L in cases of non-renal losses and >20 mmol/L in cases of renal losses.
- In euvolemic hyponatremia, evaluation for the syndrome of inappropriate antidiuretic hormone secretion (SIADH) should be performed by confirming urine osmolality >100 mOsm/kg with inappropriate urine sodium >30 mmol/L.
- Consideration of thyroid and adrenal function tests, such as TSH, free T4, morning cortisol, or ACTH stimulation test, is necessary to rule out endocrine causes.
- In hypervolemic states, assessment for heart failure, cirrhosis, or nephrotic syndrome should be conducted, with additional tests including chest X-ray, echocardiogram, or liver function tests depending on clinical suspicion.
- Medication review is crucial, as many drugs can cause hyponatremia, including thiazide diuretics, SSRIs, carbamazepine, and NSAIDs, as noted in the guidelines 1. This systematic approach helps identify the underlying cause of hyponatremia, which is essential for appropriate management, and is supported by the most recent and highest quality studies, such as those published in 2018 1.
From the Research
Investigation for Hyponatremia
To investigate for hyponatremia, the following steps can be taken:
- Take a detailed history and physical examination, including volume-status assessment 2, 3, 4, 5, 6
- Measure serum sodium level to confirm hyponatremia (less than 135 mEq/L) 2
- Assess extracellular volume status on physical examination to categorize patients as having hypovolemic, euvolemic, or hypervolemic hyponatremia 2, 3, 4
- Measure effective serum tonicity (serum osmolality less serum urea level) to determine the pathogenesis of hyponatremia 4
- Measure urine osmolality to determine whether water excretion is normal or impaired 4
- Measure urine sodium level to determine the presence of hypovolemia or the syndrome of inappropriate antidiuretic hormone secretion 4
- Determine levels of hormones (thyroid-stimulating hormone and cortisol) and arterial blood gases in difficult cases of hyponatremia 4
Laboratory Evaluation
The laboratory evaluation of hyponatremia includes:
- Serum osmolality to determine the effective serum tonicity 4
- Urine osmolality to determine whether water excretion is normal or impaired 4
- Urine sodium level to determine the presence of hypovolemia or the syndrome of inappropriate antidiuretic hormone secretion 4
- Serum urea level to determine the effective serum tonicity 4
Classification of Hyponatremia
Hyponatremia can be classified into three categories based on volume status:
- Hypovolemic hyponatremia: characterized by low urine sodium level (less than 20 mmol/L) and signs of volume depletion 4
- Euvolemic hyponatremia: characterized by normal urine sodium level and no signs of volume depletion or excess 4
- Hypervolemic hyponatremia: characterized by high urine sodium level (greater than 40 mmol/L) and signs of volume excess 4