Which test is used to differentiate the causes of Cushing's syndrome after initial screening?

Which test is used to differentiate the causes of Cushing's syndrome after initial screening?

• A. ACTH levels
• B. High-Dose Dexamethasone Suppression Test
• C. 24 Hr Urinary Free Cortisol Levels
• D. Low-Dose Dexamethasone Suppression Test

Answer: (B)

After confirming Cushing's syndrome with screening tests, the next step is to identify the source of the excess cortisol. The high-dose dexamethasone suppression test is designed to differentiate pituitary-driven (Cushing disease) from non-pituitary sources of ACTH, such as ectopic ACTH production. Pituitary adenomas usually retain some sensitivity to glucocorticoid negative feedback, so giving a high dose of dexamethasone often suppresses cortisol production by a meaningful amount. In contrast, ectopic ACTH–secreting tumors produce ACTH more autonomously and do not respond to the higher dexamethasone dose, so cortisol levels remain high. Adrenal tumors causing cortisol excess typically act independently of ACTH, and they also tend not to show suppression with the high-dose dexamethasone test. In practice, if cortisol falls with high-dose dexamethasone, this supports a pituitary source; if there is little or no suppression, an ectopic ACTH source or an adrenal cause is more likely. If results are equivocal, further testing such as CRH stimulation or invasive testing with inferior petrosal sinus sampling may be pursued to confirm the source.

After confirming Cushing's syndrome with screening tests, the next step is to identify the source of the excess cortisol. The high-dose dexamethasone suppression test is designed to differentiate pituitary-driven (Cushing disease) from non-pituitary sources of ACTH, such as ectopic ACTH production.

Pituitary adenomas usually retain some sensitivity to glucocorticoid negative feedback, so giving a high dose of dexamethasone often suppresses cortisol production by a meaningful amount. In contrast, ectopic ACTH–secreting tumors produce ACTH more autonomously and do not respond to the higher dexamethasone dose, so cortisol levels remain high. Adrenal tumors causing cortisol excess typically act independently of ACTH, and they also tend not to show suppression with the high-dose dexamethasone test.

In practice, if cortisol falls with high-dose dexamethasone, this supports a pituitary source; if there is little or no suppression, an ectopic ACTH source or an adrenal cause is more likely. If results are equivocal, further testing such as CRH stimulation or invasive testing with inferior petrosal sinus sampling may be pursued to confirm the source.