Review Question - QID 100375

A dexamethasone suppression test in a patient with a pituitary ACTH-secreting adenoma yields no change in ACTH level with a low-dose and a decrease in ACTH level with a high-dose.

Cushing disease may be endogenous or exogenous in cause. A pituitary adenoma, the most common cause of endogenous Cushing disease, leads to the excess production of ACTH. This excess ACTH acts on adrenal cortex cells (particularly fasciculata cells), causing excess production of cortisol. High cortisol levels are responsible for the many manifestations of Cushing disease ("moon facies", "buffalo hump", truncal obesity, thin skin, high blood pressure, etc).

Nieman discusses the usefulness of the dexamethasone test in determining the cause of Cushing disease. The author states that dexamethasone suppression tests work because primary adrenal disease or ectopic ACTH secreting tumors do not respond to dexamethasone, while corticotrope adenomas do if enough of the drug is administered.

Bode et al. discuss the clinical finding of hirsutism in women in which the differential includes medication side-effects, polycystic ovary syndrome, adrenal hyperplasia, thyroid dysfunction, Cushing syndrome, and androgen-secreting tumors.

Illustration A outlines the diagnostic workup of a patient with suspected Cushing's.
Illustration B shows the common physical exam findings seen in Cushing's.

Incorrect Answers:
Answers 1, 2, and 3 are not typical results of a dexamethasone test.
Answer 4: No change in ACTH levels with both a low and high-dose dexamethasone administration is consistent with an ectopic ACTH source or with an adrenal tumor.