SUPPRESSIBILITY OF PLASMA ADRENOCORTICOTROPIN BY HYDROCORTISONE - POTENTIAL USEFULNESS IN THE DIAGNOSIS OF CUSHINGS-DISEASE

Repeatedly normal cortisol suppressibility bg dexamethasone in 2 patie nts with Gushing's disease led to the present study of the prevalence of this phenomenon in 58 patients with otherwise incontrovertible evid ence of Gushing's disease. Because as many as 23% of these patients ma nifested this phenomenon, we investigated the suppressibility of plasm a ACTH: ii during iv infusion of hydrocortisone, after a priming dose (7 mg), at 3 mg/h in 8 patients and 8 normal controls; and 2) for 2 h, after oral hydrocortisone, 0.25 mg/kg, in 13 patients and 16 controls . The data showed invariable suppression of plasma ACTH to less than o r equal to 10 pg/mL (less than or equal to 2.2 pmol/L after 120 min of the infusion or at 90 min after oral hydrocortisone in 16 fasting nor mal subjects given oral hydrocortisone between 0800 and 0830 h. Plasma ACTH exceeded 10 pg/mL (2.2 pmol/L) at the same times in 14/14 patien ts with active Gushing's disease, including 3 patients whose cortisol suppressibility by dexamethasone had been misleadingly normal and in 4 /7 patients with intermittent hypercortisolism. Occasional. variations in plasma cortisol elevations after the oral dose require that plasma cortisol concentration be monitored at 60 min after the oral hydrocor tisone dose, because the present evidence supports the validity of the conclusion that a plasma ACTH concentration below 10 pg/mL excludes G ushing's disease only when plasma cortisol concentration at 60 min lie s between 16 and 38 mu g/dL. Further evaluation of ACTH suppressibilit y by cortisol would be worthwhile, to confirm its potential value in f acilitating positive diagnosis of Cushing's disease when dexamethasone suppressibility seems misleading.