Cyclical Cushing's syndrome--a trap for the unwary

A woman with cyclical Cushing's syndrome due to periodic hormonogenesis from a corticotropin (ACTH) secreting pituitary adenoma is discussed. This patient presented with acute steroid-induced psychosis but she was found to have subtle cushingoid features that went undetected for two years. Laboratory evaluation for Cushing's syndrome showed incongruous results due to periodic ACTH production by the tumour. Cyclical Cushing's syndrome may be an under-recognised phenomenon and incorrect interpretation of investigative results may lead to wrong tumour localisation and inappropriate surgery. This case highlights the subtleties and complexities that one may encounter in the diagnostic evaluation of patients with Cushing's syndrome, and emphasises that the laboratory results must always be interpreted in the appropriate clinical context.