The goal of this observational study is to evaluate the diagnostic accuracy of salivary cortisol compared with serum cortisol after a 1 mg overnight dexamethasone suppression test in participants (N = 100) with adrenal incidentalomas and healthy control participants (N = 100). The main question it aims to answer is:

Are salivary cortisol and serum cortisol equal diagnostic tools for endogenous hypercortisolism?

The inclusion of healthy controls will serve as a reference population to define the central 95 % reference interval for the serum and salivary cortisol levels.

Participants will undergo routine medical examination of adrenal incidentalomas including 1 mg dexamethasone-suppression-test, 24 h urine sample, and measurement of plasma-adrenocorticotropin (ACTH). In addition to the routine biochemical tests, saliva samples will be collected the morning and evening before intake of dexamethasone as well as the following morning together with serum cortisol.

Inclusion Criteria (patients):

• age ≥ 18 years, a new diagnosis of an adrenal incidentaloma (defined as an adrenal mass > 1 cm detected when using diagnostic imaging for non-related reasons).

Inclusion Criteria (controls):

• age ≥ 18 years, no previous history of conditions involving the adrenal glands

Exclusion Criteria (patients):

• active cancer; an adrenal incidentaloma suspected of malignancy; bilateral adrenal incidentalomas; pregnancy and breastfeeding; conditions or medical treatments that interfere with dexamethasone metabolism and cortisol metabolism/measurements; severe comorbidities; chronic kidney disease (defined as an estimated glomerular filtration rate < 60 ml/min); and inability to provide written informed consent.

Exclusion Criteria (controls):

• active cancer; pregnancy and breastfeeding; conditions or medical treatments that interfere with dexamethasone metabolism and cortisol metabolism/measurements; severe comorbidities; chronic kidney disease (defined as an estimated glomerular filtration rate < 60 ml/min); and inability to provide written informed consent.

The patient group were patients newly diagnosed with an adrenal incidentaloma and referred to either the Department of Nephrology and Endocrinology, Copenhagen University Hospital, Rigshospi-talet, Denmark or Department of Medicine, Copenhagen University Hospital Herlev Gentofte, Herlev, Denmark.

The controls were recruited through social media, online participant recruitment platform , the hospital website, and physical posters in public spaces around Copenhagen, Denmark.