Polyuria is a subjective state although a daily loss of > 2.5 L urine with persistent urine osmolalities < 300 mOsm/kg may be considered abnormal. The first line of investigation is to ascertain whether baseline values for urine volumes and plasma osmolality and sodium concentration are in fact abnormal. The next step is to determine if the increased urine production is driven by osmotically active substances excreted in the urine which cause obligate fluid loss eg glucose, ketones. It is then necessary to check if the water loss is due to either intrinsic tubular dysfunction or due to metabolic factors affecting tubular function eg hypokalaemia or hypercalcaemia. Polyuria is an infrequent manifestation of hyperthyroidism although a proportion of patients do complain of excessive thirst.
Often the most difficult patients to diagnose are those with dipsogenic polydipsia. Many of these patients are investigated with water deprivation tests which are characterised by fluctuating urine volumes and osmolalities which mirror their illicit drinks during the test. It is important to exclude those drugs which cause dryness of the mouth as a cause of increased fluid intake.
The hypertonic saline infusion test is included in this protocol for completion. However, this is a potentially dangerous procedure and should probably only be performed in an experienced specialist unit.