Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a disorder characterized by excessive release of antidiuretic hormone despite normal or low plasma osmolality, leading to water retention, dilutional hyponatremia, and impaired free-water excretion. Etiologic factors include central nervous system disorders, pulmonary diseases, malignancies—particularly small cell lung carcinoma—various medications, endocrine abnormalities, and idiopathic causes. The pathogenesis involves uncontrolled ADH-mediated activation of renal V2 receptors, resulting in increased aquaporin-2 expression and enhanced water reabsorption. Clinically, SIADH presents with euvolemic hyponatremia manifested by headache, nausea, confusion, seizures, or coma depending on the severity and rate of sodium decline. Diagnosis relies on laboratory findings of low serum osmolality, inappropriately concentrated urine, elevated urine sodium, and exclusion of adrenal, renal, and thyroid dysfunction. Treatment includes fluid restriction, correction of serum sodium with hypertonic saline in severe cases, and use of vasopressin receptor antagonists. Preventive measures focus on early identification of high-risk patients, careful use of ADH-potentiating drugs, and regular electrolyte monitoring. SIADH remains a clinically significant endocrine disorder requiring prompt recognition and controlled management to prevent neurological complications.

ADH hypersecretion; SIADH; antidiuretic hormone; hyponatremia; euvolemic hyponatremia, vasopressin; V2 receptor.