Fig. 5.24 
Vasopressin measurements found in the four types of SIAD. 
Type I SIAD accounts for some forty per cent of cases and is characterised by excessive and erratic vasopressin secretion unrelated to changes in plasma osmolality. Type II SIAD (‘reset osmostat’) is the second most common variant Patients continue to regulate water excretion about a lowered plasma osmolality. Type III SIAD is characterised by normal osmoregulation of vasopressin except when the plasma is hypotonic when there is constant and non suppressible vasopressin secretion. Type IV SIAD accounts for less than ten per cent of cases. Osmoregulation of vasopressin secretion is entirely normal and yet patients fulfil the criteria for SIAD.