Dexamethasone suppression testing in chronic renal failure: pharmacokinetics of dexamethasone and demonstration of a normal hypothalamic-pituitary-adrenal axis.

The status of the hypothalamic-pituitary-adrenal axis in chronic renal failure (CRF) was examined by dexamethasone suppression testing (DST) using oral overnight, oral and iv 8-h daytime, and standard 48-h oral dosage protocols. Based on data obtained after iv administration of dexamethasone, the daytime study was used to calculate pharmacokinetic parameters for dexamethasone (clearance, volume of distribution at steady state, and terminal t1/2). None of a group of seven uremic patients had suppressed plasma cortisol concentrations after administration of 1 mg dexamethasone, orally, the night before. Six normal subjects and six patients with CRF participated in the pharmacokinetic study. There was no significant difference between the groups with respect to clearance, volume of distribution at steady state, or t1/2 of dexamethasone, indicating that patients with CRF metabolize dexamethasone in a fashion similar to that of normal subjects. Daily patterns of plasma cortisol determined between 0800-1600 h on a day when dexamethasone was not administered were similar in normal subjects and CRF patients. However, the degree of suppression of plasma cortisol after dexamethasone was significantly greater in the normal subjects (P less than 0.01), possibly due to a prolonged cortisol t1/2 in CRF. Nevertheless, the CRF patients did have decreased plasma cortisol levels from 4-8 h after iv and from 4-7 h after oral dexamethasone. The bioavailability of dexamethasone was not significantly different between the groups. When 48-h oral DSTs were performed in the CRF group, four of five patients had normal responses. The one patient who did not suppress had low levels of plasma dexamethasone, presumably due to decreased gastrointestinal absorption of dexamethasone. These results indicate that the metabolism of dexamethasone is similar in CRF patients and normal subjects, that normal suppression of plasma cortisol can be achieved in uremia if the duration of dexamethasone administration is prolonged sufficiently to compensate for the prolongation of cortisol t1/2 in CRF, and that it is essential to measure plasma dexamethasone as well as cortisol levels to interpret the results of a DST in CRF patients.