Profile, mean residence time of ACTH and cortisol responses after low and standard ACTH tests in healthy volunteers.

OBJECTIVE: No consensus exists until now about the suitable dose of tetracosactin in the ACTH stimulation test for detecting adrenal insufficiency. Our aim was to characterize both the ACTH(1-24) and the cortisol profiles after standard high-dose test (250 microg) (HDT) and low-dose test (1 microg) (LDT) in healthy subjects in order to provide a deeper knowledge about the relationship between stimulus and response.

DESIGN AND PATIENTS: ACTH tests were performed in 10 healthy volunteers (five men, five women) with at least 1 week of difference.

MEASUREMENTS: Plasma ACTH(1-24) and ACTH(1-39) and serum cortisol were measured before tetracosactin i.v. injection and at 5, 15, 30, 45, 60, 75 and 90 min after stimulus. Area under the curve (AUC) of ACTH(1-24) and cortisol, as well as mean residence time (MRT) for ACTH(1-24) were calculated in both tests.

RESULTS: Elimination of ACTH(1-24) was faster in HDT than in LDT (MRTs of 0.14 vs 0.37, respectively, P = 0.008), but plasma concentrations were higher up to 60 min cortisol production in HDT reaching a higher maximum concentration (Cmax: 1144 vs 960 nmol/l) but delayed in time (75 vs 52.5 min). No significant relationship was observed between AUC or Cmax of ACTH(1-24) and AUC, Cmax and increment of cortisol in any of the tests. However, a negative correlation of basal cortisol values was observed with relative cortisol increment (HDT: r = 0.77 P = 0.009; LDT: r = 0.94 P < 0.0001), but not so with Cmax (HDT: r = 0.22 P = 0.55; LDT: r = 0.57 P = 0.09).

CONCLUSIONS: The elimination rate of ACTH in healthy volunteers was significantly lower in LDT than in HDT, but cortisol production rate appears to be identical in both tests, so that a maximum adrenal stimulation seems to exist. The use of LDT may be more adequate, although data from patients need studying.