Diabetes insipidus
Diagnosis

This test measures the changes in body weight, urine output, and urine composition when fluids are withheld to induce dehydration. The body's normal response to dehydration is to conserve water by concentrating the urine. Those with DI continue to urinate large amounts of dilute urine in spite of water deprivation. In primary polydipsia, the urine osmolality should increase and stabilize at above 280 mOsm/kg with fluid restriction, while a stabilization at a lower level indicates diabetes insipidus. Stabilization in this test means, more specifically, when the increase in urine osmolality is less than 30 Osm/kg per hour for at least three hours. Sometimes measuring blood levels of ADH toward the end of this test is also necessary, but is more time-consuming to perform.
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