Question 285

Question:

On a routine physical exam, a 25 year old woman admits to drinking “a lot of water.” Upon questioning, she says she drinks the equivalent of about 15 L of water a day; she has to urinate about every hour. She says she has felt thirsty ever since a car accident three years ago when “I lost a lot of blood and hit my head.” A routine blood chemistry shows a serum sodium of 144. You decide to perform a water deprivation test. Which of the following lab results after the test would suggest the patient does not have central diabetes insipidus?

Correct Answer: B. Elevated levels of ADH

Patients with central diabetes insipidus (DI) have functionally low levels of antiduretic hormone (ADH). In order to maintain a normal plasma osmolarity, they must increase their intake of free water. The other condition that can present similarly is primary polydipsia, which is a psychiatric “addiction to water.” A water deprivation test can differentiate between the two conditions.

Patients with central DI have no ADH release from the posterior pituitary despite physiological stimuli. Thus after a test they will have abnormally low levels of ADH, low urine osmolarity, elevated plasma osmolarity, and hypernatremia.

A patient with primary polydipsia will have increased levels of ADH, increased urine osmolarity, normal plasma osmolarity, and normal sodium levels (ADH helps to hold onto free water).