Endocrine Abstracts

The 1-mg overnight dexamethasone suppression test (DST) is a screening test for endogenous Cushing’s syndrome (CS). The principle is that dexamethasone (D) will suppress ACTH and cortisol (C) secretion in healthy individuals, but not in CS. We have studied the associations between D and C concentrations in patients without CS after 1-mg DST.

Methods: On suspicion of CS, 59 patients received 1-mg D at 23 p.m. and next day at 8 a.m. blood was drawn for C and D. Of 56 subjects with successful measurements, three were diagnosed with CS., C (reference 8 a.m. 142–651 nmol/l) and D were measured with in-house methods. We aimed to investigate if there was a cut-off for plasma D below which C was not suppressed using polynomial regression. Data were analyzed using Stata version 10.1 (StataCorp, etc.). The study was exempted from ethical review by the regional medical ethics committee and approved without need for informed consent.

Results: Median post-test C in the 53 subjects without CS was 48 nmol/l (IQR 28–81) (to convert to μg/l, divide by 2.548). Mean post-test D was 11.7 nmol/l (SD 5.9; range 0.8–25.7). Three subjects had D<5.0 nmol/l, and in these, C was not suppressed. Among 50 subjects with -D>5.0 nmol/l, no association of D with C was found. In the regression model, post-test D was associated with C (P<0.001 Figure 1A), and the regression line suggested a strong negative association when D was <5.0 nmol/l, but no association when D was >5.0 nmol/l (P=0.55; Figure 1A).

Conclusions: This study of the 1-mg DST suggests that 8 a.m. plasma dexamethasone <5.0 nmol/l is insufficient to suppress serum cortisol. Among people who achieved 8 a.m. plasma dexamethasone >5 nmol/l, there was no association between post-test dexamethasone and cortisol.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This work was supported, however, funding details unavailable.