What examinations should be done in a patient with hyponatremia (low sodium)?

From the Guidelines

When evaluating a patient with low sodium (hyponatremia), the most critical examinations to perform are a thorough physical examination to assess volume status, and comprehensive laboratory tests including serum osmolality, urine sodium, urine osmolality, thyroid function tests, morning cortisol, and kidney function tests. These examinations are crucial in determining the underlying cause of hyponatremia, which can be due to true sodium deficiency, water excess, or pseudohyponatremia. The pattern of laboratory results will indicate if the cause is renal sodium loss, extra-renal sodium loss, or water retention, guiding appropriate treatment. Key laboratory tests include:

• Serum electrolytes (including calcium and magnesium) 1
• Blood urea nitrogen, serum creatinine, fasting blood glucose, lipid profile, liver function tests, and thyroid-stimulating hormone 1
• Urine sodium and urine osmolality to assess the kidneys' ability to concentrate or dilute urine
• Thyroid function tests (TSH, free T4) to rule out hypothyroidism
• Morning cortisol to rule out adrenal insufficiency It is also essential to evaluate for heart failure, cirrhosis, or nephrotic syndrome, which can cause dilutional hyponatremia, and consider syndrome of inappropriate antidiuretic hormone secretion (SIADH) 1. If SIADH is suspected, chest imaging may be needed to rule out underlying malignancy. A thorough medication history should also be taken to identify any drugs that can cause hyponatremia, such as diuretics, antidepressants, and anticonvulsants. Ultimately, the goal of these examinations is to determine the underlying cause of hyponatremia and guide appropriate treatment to improve patient outcomes in terms of morbidity, mortality, and quality of life.

From the Research

Examinations for Hyponatremia

To diagnose and manage hyponatremia, several examinations and tests should be performed, including:

• Measurement of serum sodium level to confirm hyponatremia 2, 3, 4, 5, 6
• Assessment of extracellular volume status to categorize patients as hypovolemic, euvolemic, or hypervolemic 2, 3, 4, 5, 6
• Measurement of effective serum tonicity (serum osmolality less serum urea level) to determine the pathogenesis of hyponatremia 5
• Urine osmolality to determine whether water excretion is normal or impaired 5
• Urine sodium level to differentiate between hypovolemia and the syndrome of inappropriate antidiuretic hormone secretion 5
• Hormone levels (thyroid-stimulating hormone and cortisol) and arterial blood gases in difficult cases of hyponatremia 5
• Imaging studies (e.g., US) to evaluate for underlying causes of hyponatremia, such as liver disease or heart failure 6

Laboratory Evaluation

The laboratory evaluation of hyponatremia should include:

• Serum electrolyte panel to assess sodium, potassium, and chloride levels 2, 3, 4, 5, 6
• Serum osmolality to determine the effective serum tonicity 5
• Urine electrolyte panel to assess sodium, potassium, and chloride levels 5
• Urine osmolality to determine whether water excretion is normal or impaired 5

Physical Examination

A physical examination should be performed to assess:

• Extracellular volume status to categorize patients as hypovolemic, euvolemic, or hypervolemic 2, 3, 4, 5, 6
• Signs and symptoms of hyponatremia, such as weakness, nausea, headache, and neurocognitive deficits 2, 3, 4