Central diabetes insipidus is characterised by-

Central diabetes insipidus is characterised by deficient production of ADH by the hypothalamus. In pituitary, gestational, or nephrogenic DI, there is polyuria (5-20 L or more in 24 hrs)and the urine is of low specific gravity and osmolality. The polyuria results in a small (1-2%) decrease in body water and a commensurate increase in plasma osmolarity and sodium that stimulates thirst and a compensatory increase in water intake. Thus in diabetes insipidus there is low osmolality of urine (below 600 mOsm/kg) in the presence of high plasma osmolality (greater than 300 mOsm/kg). The most common d/d is primary polydipsia where the urine is excessively dilute with low plasma osmolality. Reference : page 794 Davidson's Principles and practice of Medicine 22nd edition