Question: What is cyclic Cushing’s?
Answer: Cyclic Cushing’s syndrome relates to the fact that the tumors responsible for Cushing’s syndrome, whether they are of pituitary, adrenal, or ectopic origin, produce their hormones in an irregular fashion. The secretion of ACTH or of cortisol is not always constant, but can rapidly fluctuate between high levels and low levels. This occurs over various time periods ranging from 24 hours to several days or even weeks. These fluctuations can occur with any cause of Cushing’s syndrome and can complicate the interpretation of test results. To avoid errors in diagnosis, it is usually recommended to collect several 24 hr urine samples for measurement of urinary free cortisol. Some patients with clinical manifestations suggestive of Cushing’s syndrome may have normal levels if they are measured only once or twice and they have cyclic Cushing’s syndrome. Similarly, the suppression of cortisol during a dexamethasone suppression test could be falsely determined as normal if a patient with cyclic Cushing’s is tested during a phase where the secretion rates were decreasing spontaneously.
Several cases of cyclic Cushing’s have been reported in the medical literature. In these case reports, some of the variations in cortisol levels are quite dramatic, varying from completely normal levels on one day, to very high levels on other days. It has also been observed that if 24 urine samples are collected and measured over many days, many Cushing’s patients will show quite a bit of day to day variation in cortisol levels. However, in most cases, the variations are not as dramatic as indicated in the individual case studies reported in the literature.
By Dr. Andre Lacroix MD (Fall, 1997)