Thirst

The thirst mechanism is a very basic physiologic response to maintain water balance.  

The sensation of thirst can be stimulated by:

• An increase of only 2-3% of plasma osmolality above basal levels
• Hypovolemia
• Hypotension

History

The most common cause of thirst is dehydration. Determine:

• Duration of thirst
• Type and amount of physical activity during this time
• The environment to which he/she has been exposed
• Medications that may contribute to this (i.e diuretics for hypertension, weight loss, etc.)  

Associated symptoms may include:

• Fatigue

• Weakness

• Muscle cramps

• Postural dizziness

• Decreased urine output

Physical findings consistent with dehydration include:

• Tachycardia
• Orthostatic hypotension
• Dry mucous membranes
• Decreased skin turgor
• Weight loss

Laboratory findings that would correlate with dehydration include:

• Increased urine specific gravity and osmolarity

• Increased BUN/Creatinine ratio of > 20:1

• Increased blood bicarbonate (due to metabolic alkalosis)

• Increased uric acid

• Fractional excretion of sodium <1%

Treatment involves volume replacement and minimizing further volume losses.  

• Replace volume orally with water to satisfy the thirst response.  
• If that is not enough, place an IV and give a normal saline bolus.
• Stop all medications that may be contributing to dehydration.

Other causes of increased thirst

Diabetes Insipidus

• This is due to either decreased ADH production (central) or ADH ineffectiveness at the kidney (nephrogenic).  
• Patients present with polyuria and have hypotonic urine (unlike dehydration, which has hypertonic urine). 
• Water restriction will lead to hypernatremia due to uncontrolled free water loss and continued polyuria.  
• If you suspect that a patient has this, a referral to an internist or nephrologist for diagnosis is appropriate.  
• Until a diagnosis is made, treatment would include liberal fluid intake to replace free water.

Psychogenic Polydipsia

• These patients have a compulsive water consumption that results in polyuria with hypotonic urine.  
• Often this is caused by psychiatric illness
• Medications may enhance the thirst sensation by causing dry mouth (i.e anti-cholinergics).  
• Unlike Diabetes Insipidus, water restriction will not lead to hypernatremia and will cause a decrease in urine output.  
• Referral is appropriate to confirm the diagnosis.

This section provided by LT Glen M. Arluk, MC, USNR, Naval Medical Center, Portsmouth VA

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*This web version is provided by The Brookside Associates, LLC.  It contains original contents from the official US Navy NAVMED P-5139, but has been reformatted for web access and includes advertising and links that were not present in the original version. The medical information presented was reviewed and felt to be accurate in 2001. Medical knowledge and practice methods may have changed since that time. Some links may no longer be active. This web version has not been approved by the Department of the Navy or the Department of Defense. The presence of any advertising on these pages does not constitute an endorsement of that product or service by either the US Department of Defense or the Brookside Associates. The Brookside Associates is a private organization, not affiliated with the United States Department of Defense.

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