Why does SIADH causes extreme thirst?

Answer 1

Antidiuretic hormone, in addition to acting on the kidneys to reabsorb water (decreasing urine output) when the blood becomes too concentrated, also acts on the brain to produce thirst in order to replenish fluid volume. However, in SIADH, there is an oversecretion of ADH (usually due to an ectopic ADH-producing tumor), and so the body responds as though it is in severe dehydration, even if it's not. The signs/symptoms you see are decreased urine output, which results in dilutional hyponatremia, decreased hematocrit, edema and sudden weight gain, and - because ADH triggers thirst - the patient will be thirsty even though they are already over-hydrated. The treatement is water restriction. You don't want to use diuretics because they have a tendency to excrete sodium along with water, which would make the hyponatremia worse.