TOPLINE:

Urinary free cortisol (UFC) thresholds demonstrated excellent diagnostic performance in differentiating ectopic adrenocorticotropic hormone (ACTH)-dependent Cushing syndrome from pituitary Cushing syndrome. Furthermore, severe hypercortisolism combined with hypokalaemia was a strong predictor of ectopic ACTH-dependent Cushing syndrome.

METHODOLOGY:

• Lavoillotte and colleagues proposed a pragmatic non-invasive approach using the magnitude of 24‑hour UFC levels, which in a large bicentric cohort showed excellent accuracy — outperforming plasma ACTH and other biochemical tests — and stratified patients into low-, intermediate-, and high-risk groups for ectopic vs pituitary causes of ACTH‑dependent Cushing disease.
• Researchers conducted a multicentre retrospective study to validate the diagnostic performance of UFC in distinguishing ectopic ACTH-dependent Cushing syndrome from pituitary Cushing syndrome and to determine whether adding other biochemical markers such as late‑night salivary cortisol and hypokalaemia improved the diagnostic performance.
• They included 269 patients with ACTH-dependent Cushing syndrome using data from the Spanish Cushing registry. Of these, 208 (median age, 44.9 years; 75.5% women) had pituitary Cushing syndrome and 61 (median age, 59 years; 49.2% women) had ectopic Cushing syndrome.
• For all patients, 24‑hour UFC and late‑night salivary cortisol levels were measured using immunoassay; UFC levels (expressed as multiples of the assay-specific upper limit of normal) were categorised into three groups: less than 3× the upper limit (mild/moderate), 3-9.9× (intermediate), and ≥ 10× (severe); parallel categories were used for late‑night salivary cortisol levels: less than 1.5× the upper limit (mild), 1.5-8.9× (intermediate), and ≥ 9× (severe).
• Hypokalaemia was defined as serum potassium levels below the lower limit of normal according to laboratory reference ranges.

TAKEAWAY:

• Patients with ectopic Cushing syndrome had higher median UFC levels (16.6× the upper limit of normal vs 3.6× the upper limit of normal) and late‑night salivary cortisol levels (9.3× the upper limit of normal vs 1.5× the upper limit of normal) than those with pituitary Cushing syndrome (P < .001 for both).
• The prevalence of ectopic Cushing syndrome was 12.5% in those with intermediate hypercortisolism and 40.5% in those with severe hypercortisolism; however, no cases were reported in those with mild hypercortisolism.
• Both late‑night salivary cortisol (area under the curve [AUC], 0.92; 95% CI, 0.86-0.98) and UFC (AUC, 0.89; 95% CI, 0.82-0.96) achieved excellent discrimination between ectopic and pituitary Cushing syndrome, and both outperformed baseline ACTH (AUC, 0.77; 95% CI, 0.66-0.89; P = .022).
• Hypokalaemia was more frequent in patients with ectopic Cushing syndrome than in those with pituitary Cushing syndrome (83.6% vs 5.0%). Combining severe hypercortisolism with hypokalaemia identified 75% of those with ectopic Cushing syndrome vs 1.9% of those with pituitary Cushing syndrome.

IN PRACTICE:

"[The study] results may support a paradigm shift towards a biochemistry-first approach rather than imaging-first in the differential diagnostic workup of ACTH-dependent CS [Cushing syndrome], particularly in patients with severe hypercortisolism," the authors wrote.

SOURCE:

This study was led by Betina Biagetti, Endocrinology & Nutrition Department, Hospital Universitario Vall d'Hebrón Research Institute, Department of Medicine, Autonomous University of Barcelona, Reference Networks, Barcelona, Spain. It was published online on April 30, 2026, in The Journal of Clinical Endocrinology & Metabolism.

LIMITATIONS:

Different local protocols, heterogeneous biochemical assays, and variable imaging quality and interpretation across institutions may have introduced minor intercentre variability. The relatively high proportion of patients with severe hypercortisolism was assessed using UFC levels, introducing selection bias. Because the Cushing registry was not a population-based registry, cases with mild or unresolved hypercortisolism may be underreported.

DISCLOSURES:

This study did not receive any funding from any sources. The authors declared having no conflicts of interest.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.