Cyclic Cushing syndrome as a clinical enigma and diagnostic dilemma in advanced endocrinology.

Cyclic Cushing's syndrome (CCS), a rare and atypical form of endogenous hypercortisolism, remains a very relevant problem from a diagnostic point of view. To confirm this disease, it is necessary to record two episodes of hypercortisolism alternating with period of normocortisolism. Intercyclic phase is characterized not only by negative laboratory test results but also by the absence of pronounced symptoms, making the pathology hidden and, presumably, undiagnosed in certain cases. Some causes remain undetermined and require further investigation. The process of diagnosing and verifying CCS is often lengthy and requires repeated testing at certain intervals. However, none of the currently available methods are 100% accurate. Therefore, a step-by-step approach with confirmation by several highly sensitive tests is recommended for the differential diagnosis of ACTH-dependent and ACTH-independent cyclic hypercortisolism. The risk of false results cannot be ruled out even when the most reliable methods available today are used. Thus, the diagnostic algorithm for each individual case will be specific and will depend on the phase of the disease, the patient's condition, and concomitant pathology. Our review summarizes current data on the mechanisms of cyclic hypercortisolism known to date and provides a comparative and critical analysis of modern diagnostic methods that help identify this pathology and indicate its origin.