SIADH

SIADH is the mirror image of diabetes insipidus: instead of too little water-saving hormone, there is too much. The body holds onto water it does not need, the blood becomes diluted, and blood sodium falls. The name spells out the core idea — secretion of antidiuretic hormone that is “inappropriate” for the body’s state.

Normally, dilute blood should switch vasopressin off. In SIADH, vasopressin keeps being released despite low osmolality, so the kidney keeps reabsorbing water. The extra water dilutes the blood and produces hyponatremia (low blood sodium). Crucially the problem is water excess, not salt loss; total body sodium is roughly normal but spread through too much water. Common causes include certain lung and brain conditions, many medicines, and tumors that secrete vasopressin on their own.

Symptoms come from the low sodium and range from nausea, headache, and confusion to seizures if it falls fast or far. Treatment centers on removing the cause and restricting fluid intake so the excess water can be cleared; severe cases may need careful sodium correction. Correcting sodium too quickly is itself dangerous, which makes this a condition handled with caution.