Association of Adenoma Size with 24-Hour Urinary Free Cortisol but Not Post-1-mg Dexamethasone Cortisol in Mild Autonomous Cortisol Secretion

Mild autonomous cortisol secretion is the most prevalent hormonal disorder identified in adrenal incidentalomas. Previous studies suggest a potential correlation between adrenal adenoma size and cortisol secretion. The aim of this study was to evaluate the associations of maximum adenoma diameter and adenoma volume with 24-hour urinary free cortisol levels and post-1-mg dexamethasone suppression test cortisol levels in patients with mild autonomous cortisol secretion. This cross-sectional observational study included 48 patients aged 18–65 years diagnosed with mild autonomous cortisol secretion. Mild autonomous cortisol secretion is defined as a serum cortisol level of ≥ 1.8 µg/dL after the 1-mg dexamethasone suppression test and a low morning adrenocorticotropic hormone. The 24-hour urinary free cortisol level was measured using liquid chromatography-tandem mass spectrometry. A radiologist blinded to hormonal test results assessed the maximum adenoma diameter and adenoma volume. The study population comprised 48 patients (women/men=43/5) with a mean age of 56.79±8.81 years. The mean maximum adenoma diameter and adenoma volume were 31.39±9.43 mm and 10.00±7.71 cm3, respectively. The mean 24-hour urinary free cortisol level was 25.34±18.21 µg/24 h. The 24-hour urinary free cortisol levels were positively correlated with both the maximum adenoma diameter and adenoma volume (ρ=0.518, p<0.001 and ρ=0.520, p<0.001, respectively), whereas no significant correlations were observed between post-1-mg dexamethasone suppression test cortisol levels and either the maximum adenoma diameter or adenoma volume. In patients with mild autonomous cortisol secretion, 24-hour urinary free cortisol levels were positively associated with maximum adenoma diameter and adenoma volume, whereas no significant associations were observed for post-1-mg dexamethasone suppression test cortisol levels. The clinical significance of these cross-sectional associations requires further investigation in large prospective longitudinal studies.