From the Guidelines
In patients with Syndrome of Inappropriate Antidiuretic Hormone (SIADH), the urine osmolarity is typically inappropriately high, exceeding 500 mosm/kg, despite the presence of hyponatremia and low serum osmolality. This is a key diagnostic feature of SIADH, as it indicates that the kidneys are retaining water inappropriately, leading to dilutional hyponatremia 1. The elevated urine osmolarity in SIADH is due to the excessive secretion of antidiuretic hormone (ADH), which signals the kidneys to reabsorb water regardless of the body's actual hydration status.
Some key characteristics of SIADH include:
- Hyponatremia (serum sodium < 134 mEq/L)
- Hypoosmolality (plasma osmolality < 275 mosm/kg)
- Inappropriately high urine osmolality (> 500 mosm/kg)
- Inappropriately high urinary sodium concentration (> 20 mEq/L)
- Absence of hypothyroidism or adrenal insufficiency or volume depletion
The diagnosis of SIADH is based on a combination of clinical and laboratory findings, including the assessment of intravascular volume status and biochemical measurements in blood and urine 1. The finding of elevated urine osmolarity (> 500 mosm/kg) in the setting of hyponatremia (< 134 mEq/L), along with other criteria, strongly supports the diagnosis of SIADH.
Management of SIADH typically involves free water restriction and, in severe cases, the use of hypertonic saline or medications such as demeclocycline, lithium, or vasopressin 2 receptor antagonists 1. However, the primary focus is on correcting the underlying cause of the SIADH and managing the patient's hydration status to prevent complications such as seizures, coma, and death.
From the Research
Urine Osmolarity in SIADH
- Urine osmolality in SIADH is typically high, often exceeding 500 mOsm/kg [(2, 3)].
- A study found that patients with SIADH had a mean urine osmolality of 274±78 mOsm/kg, which increased to 505±87 mOsm/kg after treatment with mild water restriction 3.
- Another study reported a mean urine osmolality of 340±100 mOsm/kg in patients with SIADH, which increased to 490±151 mOsm/kg after treatment with mild water restriction and urea 3.
- Urine osmolality can be used to guide treatment in SIADH, with patients having an initial urine osmolality lower than 400 mOsm/kg being more likely to respond to moderate water restriction alone 3.
- A case report described a patient with SIADH who had a urine osmolality of 130 mOsm/kg 4.
- High urine osmolality (>100 mOsm/kg) is one of the diagnostic criteria for SIADH, along with hyponatremia, hypo-osmolality, and high urine sodium concentration [(4, 5)].
- Urine osmolality can also be elevated in other conditions, such as central hypoadrenalism, which can present with similar symptoms to SIADH 6.