Question

About diabetes insipidus, all of the following are true EXCEPT:

• A. The water deprivation test is diagnostic
• B. Before doing the test, hypoadrenalism (cortisol deficiency) should first be corrected
• C. Hypothyroidism does not affect the diagnostic test
• D. Pre-test serum osmolality of 288 mOsm/kg is in the normal range

Hint:

Like cortisol, thyroid hormone is permissive for free-water excretion.

Answer 1

The Correct Option is C

Because the stem is phrased as all-true-except, the task is to expose the single incorrect statement while confirming the other three. Start with what is genuinely correct. The water deprivation test, judged by urine concentration and the subsequent response to desmopressin, is indeed the established diagnostic manoeuvre for diabetes insipidus. It is also standard teaching that glucocorticoid deficiency must be replaced before testing, since cortisol is needed for the kidney to excrete a free-water load and its absence can blunt or hide the polyuria of DI. A serum osmolality reading of 288 sits comfortably inside the normal band of roughly 275 to 295, so that statement holds too. The remaining option claims hypothyroidism is irrelevant to the test, but thyroid hormone, like cortisol, is permissive for free-water clearance; untreated hypothyroidism can distort the water deprivation result. That statement is therefore the false one the question is hunting for.
$ \text{cortisol and thyroid hormone both required for normal free-water excretion} $
\[\boxed{\text{Hypothyroidism does not affect the diagnostic test (FALSE statement)}}\]