How is Cushing’s Disease Diagnosed?

Diagnosing Cushing’s disease involves a series of tests to confirm excessive cortisol production and determine its source. The diagnostic process typically includes:

Initial Screening Tests

1. 24-hour urinary free cortisol test: This involves collecting urine over a 24-hour period to measure cortisol levels 1,2.
2. Late-night salivary cortisol test: Saliva samples are collected in the late evening to check for abnormally high cortisol levels when they should normally drop 1,2.
3. Low-dose dexamethasone suppression test: A small dose of dexamethasone is given at night, and blood cortisol levels are measured the next morning 1,3.
4. Blood tests: Morning blood samples are taken to measure ACTH and cortisol levels 5.

Confirmatory Tests

If initial tests suggest Cushing’s syndrome, further tests are conducted to confirm the diagnosis and locate the source:

1. High-dose dexamethasone suppression test (HDDST): This test helps differentiate between pituitary and ectopic sources of ACTH 2.
2. CRH stimulation test: Corticotropin-releasing hormone is administered to observe the response in ACTH and cortisol levels 2.
3. Inferior petrosal sinus sampling (IPSS): This invasive procedure involves collecting blood samples directly from the pituitary gland to measure ACTH levels 6.

Imaging Studies

1. MRI scan: Used to visualize the pituitary gland and detect potential tumors 1,6.
2. CT scan: May be used to examine the adrenal glands or look for ectopic sources of ACTH 1,8.

Diagnosis of Cushing’s disease can be challenging and may require multiple tests and careful interpretation by experienced endocrinologists. The process aims to first establish the presence of Cushing’s syndrome and then determine if a pituitary tumor (Cushing’s disease) is the cause.