Diagnosis. In order to distinguish DI from other causes of excess urination, blood glucose levels, bicarbonate levels, and calcium levels need to be tested. Measurement of blood electrolytes can reveal a high sodium level. Urinalysis demonstrates a dilute urine with a low specific gravity. Urine osmolarity and electrolyte levels are typically low.
A fluid deprivation test helps determine whether DI is caused by excessive intake of fluid (primary polydipsia); a defect in ADH production or a defect in the kidneys’ response to ADH.
If central DI is suspected, testing of other hormones of the pituitary, as well as magnetic resonance imaging, is necessary to discover if a disease process (such as a prolactinoma, or histiocytosis, syphilis, tuberculosis or other tumour or granuloma) is affecting pituitary function. Most people with this form have either experienced past head trauma or have stopped ADH production for an unknown reason.
Habit drinking (in it’s the severest form termed psychogenic polydipsia) is the most common imitator of DI at all ages. While many adult cases in the medical litreature are associated with mental disorders, most patients with habit polydipsia have no other detectable disease.