❑ Urinary tract infection
❑ Diabetes mellitus
❑ Diuretic therapy
❑ Bladder outlet obstruction
❑ Nephrogenic diabetes insipidus
❑ Central diabetes insipidus
❑ Primary polydipsia
Polyuria is output of 3 L or more daily, and should be differentiated from urinary frequency and nocturia. Nocturia is most commonly due to drinking fluids before sleep, but is also associated with congestive heart failure, cirrhosis, nephrotic syndrome, chronic renal failure and diuretics. Mechanisms of polyuria include: loss of renal concentrating ability (parenchymal disease); decreased bladder capacity; solute diuresis of glucose (diabetes), urea (hypercatabolic states), mannitol, or radiocontrast; postobstructive and post-ATN nephropathy; and decreased responsiveness of the tubule to aldosterone with sodium diuresis (cystic renal disease, Bartter syndrome, or resolving ATN).
Urinary tract infection Infection causes frequency of urination with irritative symptoms, but it does not increase urine volume.
Diabetes mellitus Undiagnosed or poorly controlled diabetes may cause excessive thirst and glycosuria, which is easily detected on urine dipstick. Blurred vision and weight loss are additional findings.
Diuretic therapy The cause is evident from the medication history, but hidden sources, such as diet pills or alcohol, must be sought.
Bladder outlet obstruction The prototypical cause is benign prostatic hypertrophy. There is a sensation of incomplete voiding, and frequent passage of small amounts of urine.
Nephrogenic diabetes insipidus The key to diagnosis is looking for the protean underlying conditions, which may include renal amyloidosis, myeloma, lithium therapy, hypercalcemia, hypokalemia, medullary cystic disease, analgesic nephropathy, obstructive nephropathy, or amphotericin B.
Central diabetes insipidus The sudden onset of polyuria and excessive thirst, especially for large volumes of cold drinks, is characteristic. Causes include post-pituitary surgery, head trauma, hypoxic or ischemic encephalopathy, craniopharyngioma, metastatic cancer, and infiltrative hypothalamic lesions such as sarcoidosis, and are recognized by history or concurrent neurological findings.
Primary polydipsia Urine output is consistent with the intake of large volumes of fluid, with wide fluctuation in output and lack of nocturia. The urine is dilute. It usually occurs with anxiety, psychiatric illness or with anticholinergic medications that dry the mouth.
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Copyright Details: Field Guide to Bedside Diagnosis, Copyright © 2008 Williams & Wilkins.
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Title: Field Guide to Bedside Diagnosis Authors: David S. Smith Publisher: Lippincott Williams & Wilkins Copyright: 2007 ISBN: 0-78178-165-5 
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