The low dose dexamethasone suppression test: Effect of time of administration and dose

The dexamethasone suppression test (DST) is used to determine the sensitivity of the hypothalamic-pituitary-ad renal axis (HPA-axis) to negative feedback. Dexamethasone is usually administered at 23:00 h or at midnight but this is often not convenient. The aim of this study was to determine whether the administration of dexamethasone at 2 1:00 h as compared to 23:00 h would alter the degree to which 08:00 h plasma cortisol was suppressed the following morning. Three healthy males (mean+/-SE: 32+/-7 yr) and 3 healthy females (mean SE: 35+/-7 yr) took part in the study. Each subject was orally administered I of 3 doses of dexamethasone (0.25 mg, 0.5 mg, or 1.0 mg) on 3 separate occasions in random order at least a week apart. Each dose of dexamethasone was taken at either 2 1:00 h or 23:00 h so that each subject underwent 6 tests. The differences in cortisol suppression between times and doses of dexamethasone were assessed using Analysis of Variance. Plasma cortisol was suppressed less in response to 0.25 mg dexamethasone at both 2 1:00 h and 23:00 h as compared with doses of dexamethasone of 0.5 mg or 1.0 mg (p=0.004). Suppression of plasma cortisol in response to each dose of clexamethasone was similar regardless of the timing of clexamethasone administration (p=0.5). We conclude that in healthy subjects 0.25 mg dexamethasone suppresses plasma cortisol less than either 0.5 mg or 1.0 mg and that 0.5 mg dexamethasone suppresses plasma cortisol to a similar extent as 1.0 mg dexamethasone. Moreover, changing the administration time of dexamethasone from 23:00 h to 2 1:00 h does not effect the degree to which cortisol is suppressed in healthy subjects. (C) 2002, Editrice Kurtis.