Syndrome of inappropriate secretion of antidiuretic hormone

**Definition:** Syndrome of inappropriate antidiuretic hormone (ADH) secretion (SIADH) is characterised by continued ADH secretion, leading to hyponatremia, hypoosmolality and natriuresis. Exact prevalence is unknown. The disease has been described in all age groups. SIADH is often associated with tumours, pulmonary disease, central nervous system disorders or exposure to drugs. Occasionally, it is found in patients with adrenal, thyroid or pituitary insufficiency. The disorder is caused by gain-of-function mutations in the gene encoding the vasopressin V2 receptor. Fluid restriction is the most common treatment. The outcome is related to the underlying and associated disorders.