The clinical suspicion of Cushing's syndrome leads to an obligatory laboratory evaluation, based on measurements of cortisol and ACTH. However, basal determination of these hormones is not sufficient to confirm the diagnosis, so that dynamic tests based on stimulation or inhibition of the hypothalamus-pituitary-adrenal axis (HPA) are necessary. An adequate interpretation of the laboratory results needs the understanding of the basic principles that regulate the HPA axis, the media and the form (bound or free) where the measurement will be perfomed, as well as the characteristics of the assays employed. The initial diagnostic phase, a documentation of the presence of endogenous hypercortisolism is done with the use of salivary, urinary or serum cortisol measurements, using samples collected with appropriate timing and/or after the use of low doses (1 mg) of dexamethasone. In a second phase we proceed to the etiologic diagnosis of Cushing's syndrome, using ACTH and cortisol measurements after higher doses of dexamethasone. The complexity of the syndrome frequently prompts the use of sophisticated functional tests, with the use of CRH and even samples collected during inferior petrosous sinus catheterization for the measurement of ACTH. We present the diagnostic scheme employed at the Division of Endocrinology and Metabolism of the Faculdade de Medicina de Ribeirão Preto for the confirmation of Cushing's syndrome and determination of its etiology.
Cushing's syndrome; Cushing's disease; Serum cortisol; Free urinary cortisol; Salivary cortisol; ACTH
