Polyuria:

Main causes for this can be the following:

  1. Diabetes Mellitus

  2. Diabetes Insipidus (2 types - cranial and nephrogenic)

  3. Psychogenic polydipsia

To tease out a differential diagnosis: 
Diabetes insipidus has Polyuria (due to a lack of ADH or a lack of response to it), Dilute urine (as opposed to concentrated urine with glycosuria in polyuria due to  Diabetes mellitus), Risk of dehydration if water deprived (as opposed to no such risk in polyuria of psychogenic polydipsia) and lastly, increased serum osmolality (as opposed to decreased serum osmolality in psychogenic poydipsia). Keep these features in mind and you will do well in a case with polyuria.

Usual scenario presented is either a patient who has had head injury and is suffering with polyuria (cranial DI) or a patient on Lithium (Nephrogenic DI) or one with hypercalcemia - for example lung CA patient or myeloma patient (again nephrogenic DI)

Urine is dilute and patient will become dehydrated if patient does not drink enough to replace losses.

Water deprivation test too may be done to diagnose this.

DI can be of 2 types:

Cranial: Due to lack of ADH (Antidiuretic hormone) – Head trauma, Sarcoidosis and Histiocytosis should be considered.

Nephrogenic: Due to resistance of the tubular receptors for ADH in the Kidneys.

To distinguish between the two types of DI, give desmopressin intranasally. Cranial DI will get better almost instantly. Nephrogenic will not. Nephrogenic DI on the other hand is treated with thiazides.