To develop a reliable suppression test for corticotropin (adrenocorticotrophic hormone, ACTH) for use in patients receiving anticonvulsant drugs, we selected hydrocortisone suppression of plasma corticosterone (B), whose secretion is regulated by corticotropin, thus eliminating the drug interference observed with the dexamethasone suppression test. By estimating an 8 AM base line plasma B concentration and administering 50 mg of hydrocortisone orally at midnight, we could evaluate its effect on plasma B suppression the following morning at 8 AM. The overnight suppression tests with 1 mg of dexamethasone and 50 mg of hydrocortisone reduced the plasma B concentration to less than 50% of the base line value and to less than 120 ng/deciliter in control subjects, but they failed to result in suppression in patients with Cushing syndrome. The 1-mg dose of dexamethasone caused no suppression in patients treated with anticonvulsants, whereas following hydrocortisone, base line plasma concentrations were reduced by more than 50%. The test appears to be as reliable a screening test for Cushing syndrome in drug-treated patients as the overnight dexamethasone suppression test is in patients not receiving drugs.
Meikle AW, Stanchfield JB, West CD, Tyler FH. Hydrocortisone Suppression Test for Cushing SyndromeTherapy With Anticonvulsants. Arch Intern Med. 1974;134(6):1068–1071. doi:10.1001/archinte.1974.00320240102013