American Board of Internal Medicine (ABIM) Certification Practice Exam

What is an important step after verifying excess cortisol production in Cushing syndrome diagnosis?

1. Measuring cortisol levels every hour

2. Assessing ACTH levels

3. Performing an abdominal ultrasound

4. Conducting metabolic panel testing

The correct answer is: Assessing ACTH levels

In the diagnosis of Cushing syndrome, once excess cortisol production has been confirmed, an important subsequent step is to assess ACTH (adrenocorticotropic hormone) levels. This is crucial because it helps to differentiate between the various causes of Cushing syndrome, which can include ACTH-dependent and ACTH-independent etiologies. In cases of ACTH-dependent Cushing syndrome, elevated levels of ACTH could indicate Cushing disease (caused by a pituitary adenoma) or ectopic ACTH syndrome (due to tumors producing ACTH outside the pituitary). Conversely, low ACTH levels would suggest an ACTH-independent cause such as adrenal adenoma or carcinoma, indicating that the adrenal gland itself is producing excess cortisol without the stimulation from ACTH. By determining ACTH levels, clinicians can narrow down the differential diagnosis and tailor further diagnostic evaluations appropriately, such as imaging studies or suppression tests, to pinpoint the specific cause of Cushing syndrome. This step not only aids in diagnosis but is also critical for guiding treatment strategies.