Operational Medicine 2001
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

Approved for public release; Distribution is unlimited.

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Operational Medicine 2001
Health Care in Military Settings

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