American Board of Internal Medicine (ABIM) Certification Practice Exam

Which test is primarily used for the diagnosis of Cushing's syndrome?

1. Dexamethasone suppression

2. 24 hour urine free-cortisol

3. Late-night salivary cortisol assay

4. All of the above

The correct answer is: All of the above

The diagnosis of Cushing's syndrome can indeed be made using various tests, each serving a specific purpose in assessing cortisol levels and confirming the presence of hypercortisolism. Dexamethasone suppression is a crucial test where the administration of dexamethasone, a synthetic glucocorticoid, is expected to suppress cortisol production in healthy individuals. However, in patients with Cushing's syndrome, this suppression does not occur, making it a helpful diagnostic tool. The 24-hour urine free-cortisol test measures the total amount of cortisol excreted in urine over a full day. Elevated levels indicate hypercortisolism, which is characteristic in patients suffering from Cushing’s syndrome, thus providing a direct measurement of cortisol levels. The late-night salivary cortisol assay capitalizes on the normal circadian rhythm of cortisol production, which typically exhibits a peak in the morning and a nadir at night. In Cushing's syndrome, this pattern is disrupted, often leading to elevated levels of cortisol in the late evening, making this a non-invasive and effective diagnostic screening tool. Each of these tests plays a role in the comprehensive evaluation of suspected Cushing's syndrome, which explains why all of them together are relevant for