THE RELATIONSHIP BETWEEN CORTISOL PRODUCTION-RATE AND SERIAL SERUM CORTISOL ESTIMATION IN PATIENTS ON MEDICAL THERAPY FOR CUSHINGS-SYNDROME

OBJECTIVE The aim was to determine the target range into which mean da ily serum cortisol should be lowered in patients on medical therapy fo r Cushing's syndrome, using isotopically estimated cortisol production rates as 'gold standard'. DESIGN Patients with Cushing's syndrome on medical treatment were given 12 ng of tritiated cortisol intravenously and a 24-hour urine collection was made in a single day. On the same day, serum cortisol was measured at 0900,1200,1500,1800, 2100, and at 2400 h in in-patients. In addition, serum cortisol was measured at the same times as above in a group of healthy volunteers. SUBJECTS Twenty -two patients on medical therapy for Cushing's syndrome were studied o n a total of 29 occasions. In addition, serum cortisol profiles were o btained in 12 healthy volunteers. RESULTS The median serum cortisol in patients with Cushing's syndrome was 400 (range 66-839) nmol/l, and i n the healthy volunteers 178 (range 137-299) nmol/l. The median isotop ic cortisol production rate in the patients with Cushing's syndrome wa s 84 mumol/24 h, range 10-343 (normal range 22-83) mumol/24 h. In the patients with Cushing's syndrome, the correlation of mean serum cortis ol to cortisol production rate was +0.77 (P < 0.001). Normal rates wer e found when mean serum cortisol levels were between 150 and 300 nmol/ l. CONCLUSIONS The aim of drug therapy for Cushing's syndrome should b e to lower the mean serum cortisol through the day into the range 150- 300 nmol/l.