siadh

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a disorder characterized by excessive release of antidiuretic hormone (ADH or vasopressin) leading to water retention and hyponatremia in the setting of normal or increased extracellular fluid volume. The result is typically euvolemic hyponatremia with preserved kidney, adrenal, and thyroid function.

Pathophysiology involves continued ADH action on the renal collecting ducts, increasing free water reabsorption and diluting serum sodium. Laboratory findings commonly include low serum osmolality (<275 mOsm/kg), inappropriately concentrated urine (urine osmolality often >100 mOsm/kg, frequently in the 300–1000 range) and elevated urine sodium (>20–30 mEq/L). Uric acid and BUN may be low.

SIADH is not a single disease but a syndrome with many causes. It is most often associated with central nervous system disorders (stroke, infection, trauma), pulmonary disease (pneumonia, tuberculosis), malignancies (notably small cell lung cancer), medications (select antidepressants, antipsychotics, anticonvulsants, chemotherapy agents), and postoperative states.

Diagnosis requires exclusion of other causes of hyponatremia, including hypothyroidism and adrenal insufficiency, and assessment of volume status. The typical pattern is hyponatremia with low serum osmolality, inappropriately concentrated urine, and euvolemia.

Management starts with fluid restriction. If hyponatremia is severe or symptomatic, hypertonic saline may be used with careful monitoring. Other approaches include correction of the underlying cause, salt tablets, loop diuretics, and, in selected patients, vasopressin receptor antagonists (tolvaptan or conivaptan). Rapid correction should be avoided to reduce risk of osmotic demyelination syndrome; target an increase of about 4–6 mEq/L in the first 24 hours and no more than 8–12 mEq/L per 24 hours in general.

Prognosis depends on the underlying condition driving SIADH. With appropriate management, hyponatremia often resolves, but recurrence is common if the precipitating factor persists.