What indicates Cushing's syndrome during a low-dose dexamethasone suppression test?

In the context of a low-dose dexamethasone suppression test, high cortisol levels indicate the presence of Cushing's syndrome. This condition is characterized by excessive cortisol production, which may result from various causes, including adrenal tumors, pituitary adenomas (Cushing's disease), or ectopic ACTH production from tumors.

The purpose of the low-dose dexamethasone suppression test is to determine whether the body's feedback mechanisms are functioning normally. Dexamethasone is a synthetic glucocorticoid that typically should suppress cortisol production. In individuals without Cushing's syndrome, administration of low-dose dexamethasone will lead to a decrease in cortisol levels due to the negative feedback mechanism on the hypothalamus and pituitary gland.

Conversely, in individuals with Cushing's syndrome, the cortisol levels remain high despite the administration of dexamethasone, indicating that the feedback mechanism is impaired. This failure to suppress cortisol production confirms the diagnosis of Cushing's syndrome.

Therefore, the presence of high cortisol levels after the low-dose dexamethasone administration is a key indicator of Cushing's syndrome.