Diabetes22 is any metabolic disorder exhibiting chronic polyuria. There are at least five forms of diabetes: diabetes mellitus type I and type II, gestational diabetes, renal diabetes, and diabetes insipidus. In most cases, the polyuria results from a high concentration of glucose in the renal tubule. Glucose opposes the osmotic reabsorption of water, so more water is passed in the urine (osmotic diuresis) and a person may become severely dehydrated. In diabetes mellitus and gestational diabetes, the high glucose concentration in the tubule is a result of hyperglycemia, a high concentration of glucose in the blood. About 1% to 3% of pregnant women experience gestational diabetes, in which pregnancy reduces the mother's insulin sensitivity, resulting in hyperglycemia and glycosuria. In renal diabetes, blood glucose level is not elevated, but there is a hereditary deficiency of glucose transporters in the PCT, which causes glucose to remain in the tubular fluid. Diabetes insipidus results from ADH hyposecretion. Without ADH, the collecting duct does not reabsorb as much water as normal, so more water passes in the urine.
Diabetes mellitus, gestational diabetes, and renal diabetes are characterized by glycosuria. Before chemical tests for urine glucose were developed, physicians diagnosed diabetes mellitus by tasting the patient's urine for sweet-ness.23 Tests for glycosuria are now as simple as dipping a chemical test strip into the urine specimen—an advance in medical technology for which urologists are no doubt grateful. In diabetes insipidus,24 the urine contains no glucose and, by the old diagnostic method, does not taste sweet.
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