Ectopic ADH Pathophysiology	

In the case of ectopic ADH syndrome, urinary sodium excretion continues, which further aggravates hyponatraemia and low serum osmolality. The mechanism responsible for the continuation of urinary sodium excretion has not yet been elucidated, and a ‘third factor’ has been proposed. Recently discovered natriuretic peptides – atrial natriuretic peptide and brain natriuretic peptide – may play a role in the development of ectopic ADH syndrome.
This ‘inappropriate ADH secretion’ brings about severe water excess, and serum osmolality and sodium concentration are decreased. This abnormality results in movement of water from the extracellular space to the intracellular space, which in turn induces water intoxication.