Cushing Syndrome vs Addison Disease: How to Tell Them Apart
Both are disorders centered on cortisol and are worked up through the hypothalamic-pituitary-adrenal (HPA) axis. The dividing line is simple but decisive: Cushing syndrome is a state of cortisol EXCESS, while Addison disease is cortisol (and aldosterone) DEFICIENCY from primary adrenal failure. Recognizing whether the clinical picture points to hormone excess or hormone deficiency is where the diagnosis begins.
How to tell them apart
| Feature | Cushing Syndrome | Addison Disease (Primary Adrenal Insufficiency) |
|---|---|---|
| Core hormonal problem | Cortisol excess (hypercortisolism) | Cortisol deficiency plus aldosterone deficiency (primary adrenal failure) |
| General clinical category | Signs of hormone excess | Signs of hormone deficiency |
| Body habitus / weight | Central obesity, moon facies, dorsocervical fat pad | Weight loss and anorexia |
| Blood pressure | Hypertension | Hypotension, with dizziness on standing |
| Glucose | Hyperglycemia/diabetes (cortisol excess) | Mild hypoglycemia (cortisol deficiency) |
| Skin findings | Purple striae and easy bruising | Hyperpigmentation (e.g., palmar creases, buccal mucosa) from elevated ACTH/MSH |
| Electrolytes | Not defined by hyponatremia/hyperkalemia; may show hypokalemia with severe hypercortisolism | Hyponatremia plus hyperkalemia — cortisol deficiency impairs free-water excretion and aldosterone deficiency causes sodium wasting with potassium retention |
| Muscle involvement | Proximal muscle weakness (myopathy) | Generalized fatigue and weakness with nausea |
| Acute emergency | No comparable acute decompensation | Adrenal crisis — a clinical diagnosis requiring immediate IV hydrocortisone |
The reasoning
First decide excess versus deficiency. Central obesity, striae, easy bruising, proximal weakness, hypertension, and hyperglycemia signal cortisol EXCESS → Cushing syndrome. Hypotension, orthostatic dizziness, weight loss, fatigue, and the electrolyte triad of hyponatremia with hyperkalemia signal cortisol/aldosterone DEFICIENCY → Addison disease, especially when hyperpigmentation points to a primary (high-ACTH) cause. For suspected Cushing, the workup is two-step: first confirm hypercortisolism with at least two positive tests, then measure ACTH to determine whether it is ACTH-dependent or independent. For suspected adrenal insufficiency, confirm low cortisol (with cosyntropin stimulation) and then use plasma ACTH to separate primary (Addison, high ACTH) from secondary/central (low ACTH) disease — a distinction that dictates whether mineralocorticoid replacement is needed. A history of another autoimmune disease (e.g., type 1 diabetes) raises the pretest probability of autoimmune Addison disease.
Key tests
- Cortisol excess confirmation (24-hour urine free cortisol, late-night salivary cortisol, and 1 mg overnight dexamethasone suppression): in Cushing syndrome cortisol is elevated and fails to suppress (cortisol >1.8 mcg/dL after dexamethasone); in Addison disease cortisol is deficient, not excessive.
- Plasma ACTH: in primary Addison disease ACTH is HIGH (driving hyperpigmentation), which distinguishes it from secondary adrenal insufficiency (low ACTH). In Cushing syndrome, a low/suppressed ACTH (conventionally <10 pg/mL, with values ~5–20 pg/mL indeterminate) points to an ACTH-independent adrenal source, whereas normal-to-high ACTH indicates ACTH-dependent (pituitary or ectopic) disease.
- Basic electrolytes and glucose: Addison disease shows hyponatremia, hyperkalemia, and mild hypoglycemia; Cushing syndrome is instead associated with hyperglycemia/diabetes.
- ACTH stimulation (cosyntropin) test: in Addison disease cortisol fails to rise appropriately, confirming adrenal insufficiency; not used for Cushing.
What they share
- Both revolve around cortisol and are evaluated within the HPA-axis framework
- Both use plasma ACTH as a pivotal test to clarify the underlying mechanism
- Both can cause muscle weakness and nonspecific complaints such as fatigue
- Both may show mood and cognitive changes and disturbed sense of well-being
Pitfalls
- Treating adrenal crisis as a lab diagnosis — it is a CLINICAL diagnosis; give IV hydrocortisone immediately rather than waiting for cortisol results.
- Calling central/pituitary causes 'Addison disease' — Addison refers to PRIMARY adrenal insufficiency only; low-ACTH central causes are secondary adrenal insufficiency, a different entity.
- Confusing secondary adrenal insufficiency with primary Addison disease: secondary insufficiency lacks hyperpigmentation (ACTH is low) and lacks hyperkalemia (aldosterone is preserved), so hyperpigmentation and hyperkalemia are your clues that the deficiency is primary.
- Forgetting to add fludrocortisone in primary adrenal insufficiency — only primary (high-ACTH) disease needs mineralocorticoid replacement; secondary disease is treated with hydrocortisone alone.
- Over-interpreting borderline ACTH in Cushing — values around 5–20 pg/mL are indeterminate, not clearly ACTH-independent; do not label an adrenal source until ACTH is convincingly suppressed.
- Skipping the two-step Cushing workup — confirm hypercortisolism biochemically before imaging, since jumping to CT can turn up incidentalomas that confuse the picture.
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.