SIADH

SIADH—also called syndrome of inappropriate antidiuresis (SIAD) in many current sources—is a free-water disorder, not simply a low sodium number. Vasopressin activity persists when plasma should be dilute, so the kidneys retain water and serum sodium falls by dilution. The patient typically appears euvolaemic because modest volume expansion triggers natriuresis; the absence of oedema does not mean there is no excess water. The bedside priority is symptom severity and correction speed. Seizures, coma, marked somnolence, vomiting, or cardiorespiratory distress require prompt 3% saline and carefully controlled correction. A stable patient still needs confirmation of hypotonicity, inappropriate urine concentration, and exclusion of endocrine, renal, medication-related, and hypovolaemic mimics before SIAD is diagnosed.