Measurement of Plasma Cortisol in the Diagnosis of Cushing's Syndrome

SUMMARY

Measurement of plasma cortisol is helpful in the diagnosis of Cushing's disease but not as a single determination. Stimulation with corticotropin, suppression with dexamethasone, and measurement in the evening as well as the morning are maneuvers that will help in the diagnosis, but all have problems of interpretation in certain patients. A combination of the last two, measurement of the diurnal variation of plasma cortisol and of plasma cortisol after suppression with dexamethasone, offers a useful combination of tests that is still simple enough to use as a screening procedure. The results of testing must still be correlated with the clinical findings and should be confirmed by further testing if the diagnosis of Cushing's syndrome is suggested.