The human body maintains a delicate balance of water and dissolved particles, a process known as fluid homeostasis. The kidneys play a primary role in this regulation, constantly adjusting the amount of water and solutes excreted to keep the internal environment stable. This continuous adjustment ensures that the concentration of electrolytes, like sodium, remains within a narrow, healthy range.
What Is Syndrome of Inappropriate ADH Secretion
The Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a condition defined by the excessive, unregulated release of Antidiuretic Hormone (ADH), also known as vasopressin. This hormone is typically released by the pituitary gland in response to high blood osmolality or low blood volume, signaling the kidneys to conserve water. In SIADH, however, ADH is released even when the blood is already dilute, meaning the secretion is inappropriate for the body’s actual needs.
The sustained presence of excessive ADH causes the collecting ducts in the kidneys to continuously reabsorb water back into the bloodstream. This excessive water retention increases the total body water without a corresponding increase in solutes. The primary consequence is dilutional hyponatremia, where the concentration of sodium in the blood falls below normal levels. This low blood sodium often leads to symptoms like confusion, nausea, or seizures in severe cases.
Measuring Urine Concentration
To assess the kidneys’ water-handling capability, clinicians often measure the concentration of dissolved substances in the urine. Urine Specific Gravity (USG) is a simple, quick laboratory test that measures the density of urine relative to the density of pure water. It serves as a practical, rapid estimate of the total concentration of solutes within the urine sample.
While urine osmolality is a more precise measurement that counts only the number of particles, USG is frequently used in clinical settings due to its convenience and speed. USG values are expressed as a ratio, with pure water having a value of 1.000. For a healthy adult, the normal range is generally between 1.005 and 1.030. A lower value, such as 1.003, indicates very dilute urine, while a higher value, such as 1.030, signifies highly concentrated urine.
Urine Specific Gravity Results in SIADH
In a patient with SIADH, the urine specific gravity is high, or inappropriately concentrated. Despite the patient’s blood being over-diluted and having low serum sodium, the kidneys are unable to excrete the large volume of dilute urine that would normally correct the imbalance. This inability is a direct result of the non-stop signaling from the excessive ADH.
The continuous action of ADH forces the kidney tubules to reclaim water from the forming urine, leaving the remaining dissolved particles behind in a smaller volume. This process concentrates the urine, causing the USG to be elevated.
The combination of a low serum osmolality and a high urine concentration is the defining physiological paradox of SIADH. The kidneys are acting as if the body is severely dehydrated, leading to the excretion of water-sparing urine. This concentrated urine confirms that the hormone-driven mechanism for water reabsorption is active and overriding the body’s need to eliminate excess fluid.
Using Specific Gravity for Diagnosis
The finding of a high urine specific gravity is one of the essential pieces of evidence used to diagnose SIADH. Diagnosing SIADH requires demonstrating euvolemic hypotonic hyponatremia, which means the patient has low serum sodium and low serum osmolality, but is not clinically dehydrated or obviously fluid overloaded. The concentrated urine, indicated by the elevated USG or a urine osmolality greater than 100 mOsm/kg, provides the necessary confirmation.
This concentrated urine helps distinguish SIADH from other causes of low blood sodium. For instance, in a patient who has simply consumed an excessive amount of water, the kidneys would respond appropriately by excreting a very dilute urine with a very low USG. Conversely, in conditions like Diabetes Insipidus, where there is a lack of ADH, the urine is extremely dilute, and the USG would be notably low, often below 1.005.
In clinical practice, USG is a rapid screening tool, but it is evaluated alongside other laboratory results for a definitive diagnosis. These results include serum sodium and osmolality, as well as urine sodium concentration, which is often elevated in SIADH. Together, these measurements paint a clear picture of the body’s internal fluid management, confirming that the concentrated urine is an inappropriate response to the already diluted blood.