Cortisol is mainly secreted by the adrenocortical fascicular and reticular zones. After cortisol enters the blood, 90% of the cortisol is bound to cortisol-binding proteins and clear proteins, and very little cortisol is in the free state. 10% of the free cortisol in the blood is excreted in the urine. As the secretion of cortisol has circadian rhythmic changes, and 24-hour urine free cortisol is not affected by circadian rhythmicity, more reflective of the secretory function of the adrenal cortex. Therefore, serum cortisol and 24-hour urine free cortisol are often used as the preferred indicators to screen for abnormal adrenocortical function. The indications for cortisol measurement are primarily to diagnose cortisolism, cortisol deficiency, or as part of a functional test to identify cortisolism and cortisol deficiency. The reference value for urinary free cortisol is 30-276 nmol/24 h. If an elevated 24-hour urinary free cortisol is present, it is commonly associated with hyperadrenocorticism, bilateral adrenocortical hyperplasia or neoplasia, and heterogenous adrenocorticotropic hormone syndrome. If it is borderline elevated, a low-dose dexamethasone test should be performed. Hyperadrenocorticism can be excluded when 24-hour urinary free cortisol is <276 nmol/L. If 24-hour urinary free cortisol is decreased, it is commonly associated with hypoadrenocorticism and adenopituitary hypoplasia.