From the Guidelines
In SIADH, there is an inverse correlation between serum osmolality and urine osmolality, where low serum osmolality is accompanied by inappropriately high urine osmolality. This correlation is a result of the excessive antidiuretic hormone (ADH) secretion, which leads to increased water reabsorption in the kidneys, causing the urine to be concentrated despite the low serum osmolality 1. The typical presentation of SIADH includes low serum osmolality (usually <275 mOsm/kg) and high urine osmolality (>100 mOsm/kg and often >300 mOsm/kg) 1. Key characteristics of SIADH include:
- Euvolemic hyponatremia
- Normal renal and adrenal function
- Absence of diuretic use
- Inappropriately high urine osmolality despite low serum osmolality The pathophysiology of SIADH is centered around the inappropriate secretion of ADH, which disrupts the normal regulation of water balance in the body, leading to the characteristic inverse correlation between serum and urine osmolality 1. Management of SIADH typically involves fluid restriction to prevent further dilution of serum sodium and osmolality, and in some cases, the use of vaptans, which are drugs that selectively block the V2-receptors of ADH in the collecting ducts, improving solute-free water excretion and serum sodium concentration 1.
From the Research
Correlation between Serum Osmolality and Urine Osmolality in SIADH
- In patients with Syndrome of Inappropriate Antidiuretic Hormone (SIADH), there is a characteristic correlation between serum osmolality and urine osmolality, where urine osmolality is inappropriately elevated (>100 mosmol/L) despite decreased serum osmolality 2, 3, 4, 5.
- This correlation is a key diagnostic criterion for SIADH, as it indicates that the body is retaining too much water due to excessive antidiuretic hormone secretion, leading to hyponatremia and hypo-osmolality 3, 5.
- The relationship between serum and urine osmolality in SIADH is also influenced by the patient's solute intake, with higher solute intake leading to increased urine volume and lower solute intake leading to decreased urine volume 6.
- In SIADH, the urine osmolality is typically higher than the serum osmolality, indicating that the kidneys are unable to dilute the urine properly due to the excessive antidiuretic hormone secretion 2, 4.
- The correlation between serum and urine osmolality in SIADH can be affected by various factors, including the patient's fluid intake, solute intake, and underlying medical conditions, making it essential to consider these factors when diagnosing and managing SIADH 2, 3, 6, 5.