From the Guidelines
Syndrome of Inappropriate Antidiuretic Hormone (SIADH) is characterized by laboratory values that include hyponatremia (serum sodium < 134 mEq/L), hypoosmolality (plasma osmolality < 275 mosm/kg), inappropriately high urine osmolality (> 500 mosm/kg), and inappropriately high urinary sodium concentration (> 20 mEq/L) 1. The diagnosis of SIADH is based on these laboratory findings, which reflect the pathophysiology of the disorder. Key laboratory values include:
- Hyponatremia, typically with serum sodium levels below 134 mEq/L
- Hypoosmolality, with plasma osmolality less than 275 mosm/kg
- Inappropriately high urine osmolality, greater than 500 mosm/kg
- Inappropriately high urinary sodium concentration, greater than 20 mEq/L
- Absence of hypothyroidism, adrenal insufficiency, or volume depletion These laboratory values are critical in distinguishing SIADH from other causes of hyponatremia, such as hypoadrenalism, hypothyroidism, or renal salt wasting 1. In addition to these key laboratory values, other findings may include low blood urea nitrogen (BUN), low uric acid levels, and normal to slightly elevated creatinine 1. The clinical assessment of intravascular volume status and biochemical measurements in blood and urine are essential in the diagnosis of SIADH, and the accuracy of a diagnostic algorithm for SIADH can approach 95% by assessing the effective arterial blood volume with the fractional excretion of urate 1.
From the Research
Laboratory Values in SIADH
The laboratory values in Syndrome of Inappropriate Antidiuretic Hormone (SIADH) include:
- Hyponatremia, which is the most frequent electrolyte disorder in SIADH 2
- Low serum sodium concentration, with a mean baseline serum sodium of 120.6 mEq/L in patients with SIADH 3
- Elevated urine osmolality, which is in excess of plasma osmolality 4
- High serum urea nitrogen concentration, which is correlated with the increase in serum sodium over 24 hours 3
- Normal renal and adrenal function 4
- Absence of edema and volume depletion 4
Diagnostic Criteria
The diagnosis of SIADH should be considered if the five cardinal criteria are fulfilled, including:
- Hypotonic hyponatremia
- Natriuresis
- Urine osmolality in excess of plasma osmolality
- Absence of edema and volume depletion
- Normal renal and adrenal function 4
Treatment and Correction of Hyponatremia
The treatment of SIADH includes:
- Fluid restriction, which is the mainstay of therapy for symptomatic patients with chronic SIADH 2, 4
- Hypertonic saline, which can be used to correct hyponatremia 2, 5, 6
- Tolvaptan, which is a vasopressin receptor antagonist that can be used to treat SIADH 3, 5, 6
- Urea, which can be used to treat SIADH 2
- Demeclocycline, which can be used to treat SIADH 2 The correction of hyponatremia should be limited to less than 8-10 mmol/liter per day to prevent osmotic demyelination 2