Cushing’s disease is a form of secondary hypercortisolism. The problem originates in the pituitary gland, not in the adrenal glands themselves, which makes it secondary by the standard endocrine classification. A small tumor (adenoma) in the pituitary overproduces the hormone ACTH, which then forces the adrenal glands to pump out too much cortisol. Because the adrenals are responding to an outside signal rather than malfunctioning on their own, the excess cortisol is considered secondary.
How Primary and Secondary Are Defined
In endocrinology, “primary” means the end organ itself is the source of the problem. Primary hypercortisolism occurs when something goes wrong directly in the adrenal glands, such as an adrenal tumor or abnormal nodular growth that independently produces cortisol. In these cases, ACTH from the pituitary is actually suppressed because the body recognizes there’s already too much cortisol circulating. Blood tests in primary cases often show ACTH levels near or below 1 pg/mL, essentially undetectable.
“Secondary” means the problem is one step upstream. In Cushing’s disease, a pituitary adenoma churns out ACTH on its own schedule, overriding the body’s normal feedback loop. The adrenal glands are structurally healthy but are being constantly stimulated to produce cortisol. ACTH levels in pituitary-driven Cushing’s disease average around 84 pg/mL in morning blood draws, a dramatic contrast to the near-zero levels seen in adrenal causes.
Cushing’s Disease vs. Cushing’s Syndrome
This is where the terminology trips people up. Cushing’s syndrome is the umbrella term for any condition that causes excess cortisol, regardless of the source. Cushing’s disease is one specific cause of Cushing’s syndrome: the pituitary adenoma variety. The symptoms are identical between them (weight gain in the midsection, a rounded face, thinning skin, easy bruising, high blood sugar), but the underlying mechanism and treatment differ.
Cushing’s disease accounts for about 60% to 70% of all endogenous (non-medication) cases of Cushing’s syndrome, making it by far the most common cause. Primary adrenal causes make up 10% to 20%, and ectopic ACTH production from tumors elsewhere in the body (most often the lungs) accounts for another 6% to 10%. Overall, Cushing’s disease is rare, affecting roughly 10 to 15 people per million each year.
Why the Pituitary Classification Matters
Knowing whether excess cortisol is primary or secondary changes everything about diagnosis and treatment. In Cushing’s disease, the target is the pituitary tumor. Surgeons typically approach it through the nose (transsphenoidal surgery) to remove the adenoma while preserving normal pituitary tissue. In primary adrenal causes, the affected adrenal gland itself needs to be removed.
The diagnostic workup hinges on ACTH levels as a first sorting step. If ACTH is elevated, the cause is either pituitary (Cushing’s disease) or ectopic. If ACTH is suppressed, the adrenals are acting on their own. To distinguish pituitary from ectopic sources, doctors use a high-dose dexamethasone suppression test. Dexamethasone is a synthetic steroid that normally tells the pituitary to stop producing ACTH. In Cushing’s disease, the pituitary adenoma still partially responds to this signal, and cortisol levels typically drop by more than 70% from baseline. Ectopic tumors, by contrast, ignore the signal entirely and cortisol stays high.
What Happens in the Body
Under normal circumstances, cortisol follows a daily rhythm. Levels peak in the early morning and drop to their lowest point around midnight. In Cushing’s disease, the pituitary adenoma keeps firing ACTH pulses at roughly the same frequency as a healthy pituitary, but it loses that circadian pattern. The result is that cortisol stays elevated around the clock, including overnight when it should be at its lowest. This relentless exposure is what drives the characteristic symptoms.
The constant ACTH stimulation also causes both adrenal glands to enlarge, a condition called bilateral adrenal hyperplasia. This can sometimes create confusion on imaging, since the adrenals look abnormal even though they aren’t the root cause. Pituitary adenomas in Cushing’s disease are often very small, sometimes too small to see on MRI, which adds another diagnostic challenge.
ACTH-Driven Hyperpigmentation
One clinical clue that points toward ACTH-dependent (secondary or ectopic) rather than primary Cushing’s syndrome is skin darkening. ACTH is produced from the same precursor molecule as melanocyte-stimulating hormone, the chemical that triggers pigment-producing cells in the skin. When ACTH is very high, it activates those same pigment receptors, causing darkening of the skin, particularly on the face, knuckles, inside the mouth, and areas exposed to friction. This hyperpigmentation is more dramatic in ectopic cases, where ACTH levels tend to be extremely high, but it can appear in Cushing’s disease as well. It never occurs in primary adrenal Cushing’s syndrome, because ACTH is suppressed in those patients.
The Quick Reference
- Primary hypercortisolism: adrenal gland tumors or nodular disease producing cortisol independently, with suppressed ACTH
- Secondary hypercortisolism (Cushing’s disease): pituitary adenoma overproducing ACTH, driving both adrenal glands to make excess cortisol
- Ectopic ACTH production: a tumor outside the pituitary (often in the lung) producing ACTH, also classified as ACTH-dependent but distinct from Cushing’s disease
So when you see Cushing’s disease described in a textbook or medical resource, it sits squarely in the secondary category. The adrenals are the victims, not the villains. The real problem is a tiny tumor in the pituitary telling them to work overtime.