Differentiating Cushing’s Syndrome by Etiology: A Stepwise Diagnostic Framework Guiding Treatment Selection

Introduction

Cushing’s syndrome (CS) is a clinical and metabolic disorder caused by pathological hypercortisolism. The key diagnostic challenge is not only to confirm hypercortisolism but also to identify its etiology, which directly determines treatment strategy, prognosis, and long-term outcomes. The three main etiologies of endogenous CS are pituitary-dependent Cushing’s disease (CD, 70–80%), ectopic ACTH syndrome (EAS, 5–10%), and ACTH-independent adrenal Cushing’s syndrome (10–15%). These entities require distinctly different treatments: transsphenoidal surgery for CD, tumor resection for EAS, and adrenalectomy for adrenal CS. Misdiagnosis or misclassification delays definitive treatment, increases comorbidity, and may be life-threatening in severe cases. This narrative review summarizes current diagnostic algorithms and treatment-guided pathways to provide a clinically actionable framework for clinicians 111.

Clinical Presentation and Initial Diagnostic Suspicion

The clinical phenotype of CS varies in severity and presentation but consistently includes core manifestations that should trigger diagnostic evaluation. Central obesity with disproportionate fat redistribution, moon facies, proximal muscle weakness, purple striae (particularly those wider than 1 cm), easy bruising, hypertension, glucose intolerance, menstrual irregularities, osteoporosis, and neuropsychiatric symptoms including mood disturbance and cognitive impairment are characteristic features22. Additional findings include thin skin, dorsocervical fat pads (buffalo hump), facial plethora, hyperpigmentation (more common in ACTH-dependent forms), and in pediatric populations, reduced growth velocity despite weight gain22.

Certain clinical features provide early clues to etiology. Severe, rapidly progressive hypercortisolism with hypokalemia suggests ectopic ACTH syndrome, which typically presents in men over age 40 with small-cell lung carcinoma, bronchial neuroendocrine tumors (25.4%), or pancreatic/gastrointestinal neuroendocrine tumors (11.8%)1122. In contrast, pituitary-dependent Cushing's disease predominantly affects women aged 25–40 years and often presents with more insidious onset11. The presence of at least one specific feature (e.g., purple striae, proximal weakness) combined with one non-specific feature (e.g., hypertension, obesity) should prompt biochemical screening22.

Biochemical Differentiation Strategy: Screening and Confirmation

No single diagnostic test provides definitive confirmation of CS; therefore, a multimodal approach is mandatory. The Endocrine Society guideline recommends initial screening using one of the recommended first-line tests, with confirmation through concordant abnormal results from additional testing when appropriate10. The three complementary first-line screening tests exploit the loss of normal circadian cortisol variation and resistance to feedback suppression characteristic of Cushing's syndrome1.

Screening Test Performance

In comprehensive evaluations, 24-hour urinary free cortisol (UFC) demonstrated the highest diagnostic accuracy with sensitivity of 96.0%, specificity of 99%, and area under the curve (AUC) of 0.988 at a cutoff of 144.6 μg/24 hours2. Late-night salivary cortisol (LNSC) at a cutoff of 10.5 nmol/L yielded sensitivity of 85.7%, specificity of 88.2%, and AUC of 0.8972. The overnight dexamethasone suppression test at a cutoff of 94.6 nmol/L provided sensitivity of 96.0%, specificity of 99.03%, and AUC of 0.9952. These data establish UFC as the best single screening test, though concordant positivity across multiple modalities improves diagnostic confidence.

However, important limitations exist. LNSC sensitivity is limited in subclinical Cushing's syndrome (82% sensitivity, 60% specificity at a cutoff of 0.18 μg/dL), potentially leading to false-negative results if used as the sole screening test3. Dexamethasone suppression testing can produce false-positive results due to altered (increased) dexamethasone metabolism in certain patients, underscoring the critical importance of concordant biochemical evidence before proceeding to invasive localization testing6.

ACTH Measurement: The Critical Fork in the Diagnostic Pathway

Once hypercortisolism is established, plasma ACTH measurement creates the first major diagnostic fork: ACTH-dependent (pituitary or ectopic) versus ACTH-independent (adrenal) disease. An elevated or inappropriately normal ACTH (typically >10 pg/mL) indicates ACTH-dependent disease, whereas suppressed ACTH (<5 pg/mL) points to primary adrenal pathology10.

However, ACTH measurement harbors critical pitfalls. Modern two-site sandwich ACTH immunoassays retain potential for interference from heterophile antibodies, pro-opiomelanocortin (POMC), and ACTH fragments4. A case series demonstrated that the Siemens ACTH Immulite assay produced erroneously elevated results leading to inferior petrosal sinus sampling and unnecessary pituitary surgery in a patient who actually had adrenal (ACTH-independent) Cushing syndrome confirmed using alternate assays4. When ACTH results are discordant with clinical presentation or biochemical context, measurement on alternative assay platforms using different antibody species and epitopes should be considered12.

Furthermore, significant overlap in plasma ACTH levels exists between patients with adrenal Cushing's syndrome and those with Cushing's disease. In a multicenter study, the high-dose dexamethasone suppression test (HDST) showed superior discriminatory performance with AUC of 0.997 (sensitivity 97.9%, specificity 96.7%) compared to plasma ACTH alone (AUC 0.954, sensitivity 84.2%, specificity 94.3%)5. The HDST is particularly useful when plasma ACTH level alone is not conclusive.

Localization and Source Identification in ACTH-Dependent Disease

Once ACTH-dependent Cushing's syndrome is confirmed, the pivotal diagnostic question is whether the ACTH source is pituitary (CD) or ectopic (EAS)—a distinction with profound treatment implications.

High-Dose Dexamethasone Suppression Testing

The HDST (8 mg overnight or 8 mg/d for 48 hours) serves as the initial discriminatory test. Pituitary corticotroph adenomas retain partial sensitivity to glucocorticoid negative feedback; thus, high-dose dexamethasone typically suppresses UFC by ≥50% from baseline in CD, whereas ectopic ACTH-secreting tumors are generally resistant to suppression10. However, approximately 10–15% of CD patients show no suppression, and some ectopic tumors (particularly bronchial carcinoids) may exhibit partial suppression, limiting the test's specificity.

Inferior Petrosal Sinus Sampling: Gold Standard with Technical Complexities

When high-dose DST results are discordant with clinical suspicion or when a clear pituitary lesion is absent on magnetic resonance imaging (MRI), inferior petrosal sinus sampling (IPSS) is indicated10. IPSS measures ACTH concentrations in blood sampled from the inferior petrosal sinuses (which drain the pituitary) and compares them to peripheral ACTH. A central-to-peripheral ACTH ratio ≥2 at baseline and ≥3 after stimulation is generally considered supportive of a pituitary source.

The diagnostic performance of IPSS is critically dependent on pharmacological stimulation. In a retrospective cohort of 40 patients with ACTH-dependent CS and negative MRI findings, unstimulated IPSS yielded sensitivity of only 85% and specificity of 57%, with negative predictive value of 44%7. After ovine corticotropin-releasing hormone (oCRH) stimulation (using standard cutoff of central-to-peripheral ratio ≥3:1), sensitivity increased to 97%, specificity to 71%, and negative predictive value to 83%7. The authors concluded that ACTH measurements without oCRH stimulation cannot be recommended and advocate for desmopressin or commercially available human CRH as alternatives during drug shortages7.

Recent data from 230 BIPSS procedures using desmopressin stimulation and prolactin measurements to confirm catheter position demonstrated sensitivity of 95.9% (without ACTH/prolactin ratio calculation) and 97.3% (with normalized ratio), with specificities of 92% and 93.8%, respectively20. Optimized protocols using desmopressin sampling at t = 0 and t = +5 minutes preserved diagnostic accuracy while reducing procedural costs from $6,392 to $2,02817.

Technical Pitfalls and False Results

Anatomical variations and vascular cross-filling present significant challenges to IPSS accuracy. In 68% of patients with confirmed ACTH-secreting microadenomas, angiographic evidence of cross-filling between inferior petrosal sinuses was documented, causing mean ACTH increase of 3.91-fold in the contralateral IPS compared with 1.80-fold in patients without cross-filling (p = 0.014)13. This vascular anatomy substantially impacts lateralization specificity and may lead to false localization.

False-negative IPSS results occur in approximately 3.5% of procedures15. Management includes screening for ectopic ACTH/CRH sources via chest/abdomen/pelvis CT and functional imaging (such as 68Ga-DOTATATE PET-CT), repeat IPSS if ectopic workup is negative, and consideration of medical cortisol-lowering therapy as a bridge until definitive diagnosis is established15.

Imaging Strategies

Pituitary MRI with contrast is the initial imaging modality for suspected CD, though microadenomas (<10 mm) remain invisible in 30–50% of cases1011. Importantly, 10% of healthy adults harbor incidental pituitary lesions ≤6 mm on MRI, and 12% of patients with EAS have abnormal pituitary imaging; therefore, MRI findings must be interpreted in conjunction with biochemical data, not in isolation10.

For ectopic ACTH syndrome, chest CT is first-line imaging. In approximately 10–15% of EAS cases, the tumor remains occult despite extensive imaging; in these situations, medical therapy or bilateral adrenalectomy becomes the primary treatment option1011. Clinicians must remain vigilant for ectopic pituitary adenomas arising in the sphenoid sinus without direct sellar involvement—a rare but diagnostically challenging entity8.

Treatment Selection Decisively Determined by Diagnostic Pathway

The etiologic diagnosis creates three distinct treatment pathways, each with specific first-line, second-line, and bridging approaches.

Pituitary-Dependent Cushing's Disease

Transsphenoidal selective adenomectomy (TSS) by an experienced pituitary surgeon is the first-line treatment for CD. Remission rates are 73–76% for microadenomas and approximately 43% for macroadenomas10. Postoperative remission is defined as morning serum cortisol <5 μg/dL or UFC <10–20 μg/d within 7 days of surgery. However, recurrence occurs in 15–66% of adults within 5–10 years, with late-night salivary cortisol appearing to be an earlier predictor of recurrence compared with urinary free cortisol excretion9.

When initial surgery fails, options include repeat transsphenoidal surgery (particularly if incomplete resection is suspected), radiation therapy or radiosurgery (especially for invasive or recurrent CD), medical therapy with steroidogenesis inhibitors or pituitary-directed agents such as pasireotide (achieving normal UFC in 20% of CD patients but causing hyperglycemia in 73%)10, and bilateral adrenalectomy for severe disease10.

Ectopic ACTH Syndrome

Surgical resection of the ectopic tumor with appropriate node dissection is curative in approximately 76% of patients without overt metastatic disease10. When the tumor cannot be localized or is unresectable, medical therapy with steroidogenesis inhibitors (ketoconazole normalizes UFC in 57–93% of patients; metyrapone controls hypercortisolemia in 50–75%)10 becomes the primary approach. In life-threatening situations with severe hypercortisolism, bilateral adrenalectomy provides rapid cortisol control10.

Adrenal-Origin Cushing's Syndrome

When ACTH is suppressed (<5 pg/mL) in the setting of hypercortisolism, primary adrenal disease is present. Unilateral adrenalectomy by an experienced adrenal surgeon is curative for benign adenomas, with success rates approaching 100%10. Bilateral adrenalectomy is indicated for bilateral hyperplasia, though this necessitates lifelong glucocorticoid and mineralocorticoid replacement. For adrenocortical carcinoma, complete resection is the goal, often followed by adjuvant mitotane therapy10.

Clinical Decision-Making in Equivocal and Severe Cases

Non-Invasive Diagnostic Models

In an effort to reduce invasive procedures, non-invasive models combining demographic, clinical, biochemical, and imaging parameters have been derived to identify patients with CD who could proceed directly to surgery without BIPSS. Models using gender, hypokalemia, plasma ACTH, HDST (≥57% cortisol suppression), and pituitary MRI provided 100% positive predictive value for CD and avoided BIPSS in 41.2% of cases16.

Cyclic Hypercortisolism

Cyclic CS—characterized by intermittent cortisol elevation—can produce false-negative screening test results, necessitating repeated UFC or midnight salivary cortisol measurements in suspected cyclic disease10. A case of cyclic ectopic ACTH secretion from a pheochromocytoma illustrates the importance of considering rare ectopic sources in ACTH-dependent disease with an adrenal mass and elevated plasma metanephrines18.

Severe Hypercortisolism and Bridging Therapy

Severe CS is commonly characterized by markedly elevated cortisol levels together with serious metabolic, cardiovascular, infectious, or neuropsychiatric complications. These patients often require urgent cortisol control to prevent life-threatening complications including hypokalemic alkalosis, severe hypertension, thromboembolism, and acute psychiatric decompensation24. Rapid cortisol suppression via medical therapy using titration or block-and-replace strategies with steroidogenesis inhibitors may be necessary while arranging definitive surgery or awaiting radiotherapy effects24.

Conclusion

Accurate differentiation of Cushing's syndrome by etiology requires a stepwise diagnostic approach integrating biochemical screening with at least two concordant tests, ACTH measurement with awareness of assay-specific interference, imaging (pituitary MRI, chest/abdomen/pelvis CT, adrenal CT), and when indicated, bilateral petrosal sinus sampling with appropriate stimulation. Each diagnostic pathway—pituitary-dependent, ectopic ACTH, or adrenal-origin disease—directs distinct treatment modalities: surgery for localized disease, medical therapy as a bridge or definitive approach, and multidisciplinary management for severe or occult disease. Misclassification risks unnecessary surgery, delayed diagnosis of ectopic tumors, and prolonged morbidity; therefore, diagnostic precision and multidisciplinary review of concordant biochemical and imaging evidence are essential before committing to invasive procedures or definitive surgical intervention.