From the Guidelines
The recommended workup for hypercortisolism (Cushing's syndrome) begins with first-line screening tests, including dexamethasone suppression testing, 24-h UFC, and late-night salivary or sleeping midnight serum cortisol level, as these tests have the highest sensitivity and specificity for diagnosing hypercortisolism 1. The initial screening should include one or more of the following:
- 24-hour urinary free cortisol measurement
- Late-night salivary cortisol testing
- The overnight 1-mg dexamethasone suppression test At least two abnormal results from different tests are typically required for diagnosis. Once hypercortisolism is confirmed, the next step is to measure plasma ACTH levels to differentiate between ACTH-dependent (pituitary or ectopic source) and ACTH-independent (adrenal) causes. For ACTH-dependent cases, a high-dose dexamethasone suppression test and pituitary MRI are performed to distinguish between pituitary adenoma (Cushing's disease) and ectopic ACTH syndrome, as suggested by recent guidelines 1. Corticotropin-releasing hormone (CRH) stimulation testing and inferior petrosal sinus sampling may be needed in ambiguous cases. For ACTH-independent hypercortisolism, adrenal imaging with CT or MRI is indicated to identify adrenal tumors or hyperplasia. This stepwise approach is necessary because Cushing's syndrome has diverse etiologies requiring different treatments, and the clinical presentation can overlap with common conditions like obesity, hypertension, and diabetes, making laboratory confirmation essential before pursuing invasive procedures or treatments 1.
From the FDA Drug Label
2.1 Laboratory Testing Prior to ISTURISA Initiation Correct hypokalemia and hypomagnesemia prior to starting ISTURISA Obtain baseline electrocardiogram (ECG). Repeat ECG within one week after treatment initiation, and as clinically indicated thereafter
- The recommended workup for hypercortisolism (Cushing's syndrome) includes:
- Correcting hypokalemia and hypomagnesemia before starting treatment
- Obtaining a baseline electrocardiogram (ECG)
- Repeating the ECG within one week after treatment initiation and as clinically indicated thereafter
- Monitoring cortisol levels from at least two 24-hour urine free cortisol collections every 1-2 weeks until adequate clinical response is maintained 2
- Additionally, for patients with Cushing's disease, the following baseline evaluations are recommended before initiating treatment with pasireotide:
- Fasting plasma glucose (FPG) and hemoglobin A1c (HbA1c)
- Liver tests
- Electrocardiogram (ECG), serum potassium, and serum magnesium levels 3
From the Research
Hypercortisolism Workup
The recommended workup for hypercortisolism (Cushing's syndrome) involves several steps:
- Excluding iatrogenic causes (glucocorticoid administration) 4
- Measuring "free" (unmetabolised) cortisol in a 24-hour urine (UFC) sample 4
- Performing a low-dose (1 or 1.5 mg) dexamethasone (Dex) test 4, 5
- Measuring plasma cortisol after dexamethasone suppression 4, 6, 5
- Measuring late-night salivary cortisol as a screening test for Cushing's syndrome 7
Differentiating Between Causes of Cushing's Syndrome
After establishing the diagnosis of Cushing's syndrome, the following tests can be used to differentiate between its pituitary, adrenocortical, or ectopic origin:
- Plasma ACTH measurement 4, 5
- Corticotropin releasing hormone injection test (with plasma ACTH/cortisol measurement) 4
- High-dose Dex (8 mg or more) suppression test 4, 5
Additional Tests
Other tests that may be used in the workup of hypercortisolism include:
- Basal morning plasma adrenocorticotroph hormone measurement 6
- 24-h urinary free cortisol measurement 6, 7
- Late-night salivary cortisol measurement 6, 7
- Scalp-hair cortisol/cortisone analysis to assess long-term glucocorticoid exposure 5
Limitations of Tests
It is essential to be aware of the limitations of these tests, including: