Posterior pituitary: diabetes insipidus
The posterior pituitary gland secretes two hormones, arginine vasopressin (AVP) and oxytocin.
Diabetes insipidus (DI) is defined as the inappropriate passage of large volumes of dilute urine (<300mOsm/L). Due to either deficiency in AVP production (cranial DI) or resistance to its actions at the kidney (neph-rogenic DI). The most common cause of DI is primary deficiency of AVP production (i.e. cranial DI). This may be acquired or inherited in origin
Children present with polydipsia, polyuria, and nocturia, which must be distinguished from more common causes. Infants may exhibit failure to thrive, fever, and constipation. Other symptoms may be related to the underlying cause, e.g. headache, visual acuity/visual field impairment.
When suspected, assessment of 24hr urinary volume and osmolality under conditions of ad libitum fluid intake should be undertaken. Serum osmolali-ty, U&E (Na+), and blood glucose should also be measured.
Blood hypertonicity (serum osmolality >300mOsm) with inappropriate urine hypotonicity (urine osmolality <300mOsm) should be demonstrated. Diabetes mellitus and renal failure should be excluded.
A water deprivation test and assessment of responses to exogenously administered ADH is required to diagnose the type of DI. Other tests to determine the underlying cause of DI will also be needed (e.g. cranial MRI imaging).