The diagnosis and differential diagnosis of endogenous Cushing's syndrome

Cushing's syndrome (CS) is a physically and psychologically disabling disease associated with high morbidity resulting from inappropriate elevation of circulating free cortisol levels. The main features of CS are disturbance of the normal circadian rhythm of cortisol secretion, impairment of the normal feedback of the hypothalamo-pituitary-adrenal (HPA)-axis, and excessive integrated 24 hours cortisol secretion. All biochemical tests used for the diagnosis of CS rely upon the ascertainment of a disturbance of these features. However, the diagnosis of CS (endogenous hypercortisolism) still remains a challenge, although the evolution of several diagnostic tests has allowed diagnosis at an earlier stage. In the initial investigation of CS, tests of high sensitivity are required to identify patients at risk, which are followed by tests of high specificity to confirm the diagnosis and establish the precise aetiology. This review will discuss the various causes of endogenous CS and focus on established and evolving diagnostic procedures used for its diagnosis, as several studies with large number of patients have recently appeared in the literature validating current practice and proposing improved diagnostic algorithms.