Cushing Disease vs Cushing Syndrome: What's the Difference?

Key takeaways

• Cushing syndrome is the broad term for excess cortisol from any cause, while Cushing disease is one specific cause of Cushing syndrome.
• Cushing disease is caused by an ACTH-secreting pituitary adenoma and is the most common cause of endogenous Cushing syndrome.
• Exogenous glucocorticoid use is a common cause of Cushing syndrome and must be ruled out before testing the adrenal-pituitary axis.
• Laboratory testing, including late-night salivary cortisol, 24-hour urinary free cortisol, and dexamethasone suppression, helps confirm Cushing syndrome before the cause is identified.
• Distinguishing ACTH-dependent from ACTH-independent causes guides imaging and treatment decisions.

Definitions: Syndrome vs Disease

Cushing syndrome is a clinical diagnosis that refers to the signs and symptoms resulting from chronic exposure to too much cortisol. The source can be outside the body, such as prescribed glucocorticoid medications, or inside the body, such as a tumor that produces cortisol or stimulates the adrenal glands to produce it. Because the syndrome is defined by the hormonal effect rather than by one specific lesion, it has many possible causes.

Cushing disease is a narrower term. It refers specifically to Cushing syndrome caused by a pituitary adenoma that secretes adrenocorticotropic hormone (ACTH). The excess ACTH then drives the adrenal glands to produce too much cortisol. In other words, Cushing disease is a subset of Cushing syndrome. Not everyone with Cushing syndrome has Cushing disease, but everyone with Cushing disease has Cushing syndrome.

Causes and Classification

Causes of Cushing syndrome are often divided into exogenous and endogenous categories. Exogenous causes come from outside the body, most commonly long-term use of glucocorticoid medications such as prednisone, dexamethasone, or hydrocortisone. These medications are prescribed for many inflammatory, autoimmune, and allergic conditions, and they can produce the full picture of Cushing syndrome even when the adrenal glands and pituitary are functioning normally.

Endogenous causes originate within the body and are classified by whether they are ACTH-dependent or ACTH-independent. ACTH-dependent causes include Cushing disease, the most common endogenous cause, and ectopic ACTH secretion from tumors outside the pituitary. ACTH-independent causes include adrenal adenomas, adrenal carcinomas, and adrenal nodular hyperplasia, in which the adrenal gland itself overproduces cortisol. Determining which category applies is a central step in evaluation.

Signs, Symptoms, and Clinical Overlap

The clinical features of Cushing syndrome are similar regardless of the underlying cause. People may experience weight gain, particularly around the abdomen and face, a rounded face sometimes called moon facies, a fatty hump between the shoulders, easy bruising, purple stretch marks, muscle weakness, and thinning skin. Other findings can include high blood pressure, high blood sugar, mood changes, and increased susceptibility to infections.

Because these features overlap with many other conditions, clinicians rely on a combination of history, physical examination, and laboratory testing. The presence of a pituitary tumor does not by itself distinguish Cushing disease from other forms of Cushing syndrome, since the symptoms are driven by cortisol excess rather than by the tumor's location. This overlap is why biochemical testing is essential before attributing symptoms to a specific cause.

Diagnosis and Laboratory Testing

The first step is usually to confirm that cortisol excess is present. Common initial tests include late-night salivary cortisol, 24-hour urinary free cortisol, and a low-dose dexamethasone suppression test. These tests assess cortisol production over time and help distinguish true Cushing syndrome from conditions that can mimic it, such as obesity, depression, or alcohol use disorder. Exogenous glucocorticoid use should be reviewed carefully, because it can cause test results that are difficult to interpret.

Once Cushing syndrome is confirmed, the next step is to measure ACTH to separate ACTH-dependent from ACTH-independent causes. A high or inappropriately normal ACTH level points toward an ACTH-dependent cause such as Cushing disease or ectopic ACTH secretion. Additional testing, such as high-dose dexamethasone suppression or inferior petrosal sinus sampling, and imaging of the pituitary or adrenal glands, may be used to localize the source. Treatment differs by cause, ranging from pituitary surgery for Cushing disease to adrenal surgery or medication for adrenal causes.

Frequently Asked Questions

Sources

• NIH: Cushing's Syndrome
• MedlinePlus: Cushing Syndrome
• NCBI: Cushing Disease
• MedlinePlus: ACTH Blood Test