What is the overall laboratory strategy for diagnosing Cushing’s disease?

Diagnosis of Cushing’s disease—the predominant endogenous form of Cushing’s syndrome—requires a laboratory strategy that sequentially rules out potential causes of excess cortisol until the correct one is identified. After ruling out exogenous causes of Cushing’s-like symptoms, the first step is to document cortisol excess. Blood cortisol concentration exhibits diurnal variation, so it is less useful than 24-hour urinary free cortisol, which the Endocrine Society recommends for demonstrating overproduction of the hormone.

The other two recommended screening tests are salivary cortisol and the 1 mg overnight dexamethasone suppression test. If two of these three screening tests are abnormal, further investigation should be undertaken.

If elevated adrenocorticotropic hormone (ACTH) is detected, this suggests an ACTH-dependent form of Cushing’s syndrome. About 30% of Cushing’s syndrome cases are ACTH-independent, but among ACTH-dependent cases, nearly all are caused by an ACTH-secreting pituitary adenoma and classified as Cushing’s disease. The remainder of ACTH-dependent cases are due to ectopic ACTH producing tumors, which are very rare and usually occur in the lungs or thymus, although they have also been described in the thyroid, ovaries, liver, and adrenal glands.

In the case of ACTH-dependent Cushing’s syndrome, a high-dose dexamethasone suppression test helps determine if the case is Cushing’s disease or caused by ectopic ACTH production. High doses of dexamethasone often suppress ACTH production by pituitary adenomas, whereas no suppression occurs in ectopic tumors. Stimulation with corticotropin releasing hormone (CRH) also tends to exaggerate ACTH release in Cushing’s disease, whereas ectopic tumors respond poorly or not at all. A third option for distinguishing between ectopic or pituitary-produced ACTH is computed tomography in combination with magnetic resonance imaging.

In a small fraction of cases, however, the results of all of these tests are non-diagnostic. When this occurs, bilateral inferior petrosal sinus sampling (BIPSS) is needed to verify that the source of ACTH is the pituitary gland.

What does the BIPSS procedure involve?

Venous catheters are first inserted in the left and right femoral veins and maneuvered into the jugular bulb, where contrast dye is injected to radiographically visualize the petrosal sinuses. The catheters are then extended into both inferior petrosal sinuses (IPS) to sample their contents. CRH is administered to stimulate the release of ACTH, which is measured in the IPS and a peripheral vein before and after stimulation.

Bilateral sampling is necessary because the petrosal sinuses may not receive equal amounts of pituitary hormones. In fact, it has been suggested that patients can have a dominant petrosal sinus into which ACTH preferentially drains.

How should the results of BIPSS be interpreted?

In patients with an ectopic ACTH producing tumor, the ratio of IPS:venous ACTH concentrations will be less than 1.4:1. In patients with Cushing’s disease, the ratio will exceed 2.0, with averages near 15. The sensitivity and specificity of BIPSS for Cushing’s disease are 88-100% and 67-100%, respectively (Endocr Connect 2016; doi: 10.1530/EC-16-0029). 

Are there alternative tests to definitively diagnose Cushing’s disease?

A less invasive procedure, jugular vein sampling (JVS), has been proposed. JVS has similar sensitivity and specificity to BIPSS, but when JVS results are negative, BIPSS is still recommended as the definitive test. Cavernous sinus sampling (CVS) has also been suggested as an alternative to BIPSS. The cavernous sinuses are in closer proximity to the pituitary and may therefore have higher concentrations of ACTH, improving the sensitivity of the test. However, CVS carries a higher risk of adverse events and is not routinely used in current practice.

Roger L. Bertholf, PhD, DABCC, FACB, is medical director of clinical chemistry at Houston Methodist Hospital in Texas. +Email:[email protected]