Background &, objectives: Cortisol response to 250mug adrenocorticotropin (ACTH) exhibits no circadian variation. Information on the circadian variation, if any, in cortisol response to 1 mug ACTH, which is considered as a physiological dose is not available. As the 1 mug ACTH stimulation test is projected as an outpatients procedure with no time constraint, this information is very important. Hence, this study was designed to assess whether any circadian variability exists in cortisal response to 1 mug ACTH in health), subjects.Methods: Thirty six healthy volunteers (23 male and 13 female) with mean age of 32.2 +/- 9.0 yr were consecutively studied after obtaining informed consent. On day 1, prestimulated and stimulated plasma cortisol samples were collected at 0800 h and, at 30 and 60 min following an intravenous bolus of 1 mug ACTH, and on day 3, plasma cortisol samples were similarly collected at 1600h. Cortisol estimation was done by a sensitive and specific radioimmunoassay. Stimulated plasma cortisol of 500 nmol/l or more was defined as a normal response.Results: The prestimulated and peak cortisol levels at 0800h (377.5 +/- 93.3 and 729.1 +/- 183.2 nmol/l) were higher (P < 0.001 and P <0.01) than those at 1600 h (230.1 +/- 75.7 and 665.8 +/- 138.6 nmol/l). However, a stimulated cortisol response of 500 nmol/l or more was observed at both 0800h and 1600h in all subjects at 30 min but not at 60 min. The A (peak-basal) response was higher at 1600h than that at 080011 (432.8 +/- 136.8 vs 351.5 +/- 177.3, P <0.01).Interpretation & conclusion: The demonstration of normal cortisol response to 1 mug ACTH both at 0800h and 1600h suggests that the test can be performed at any time of the day.

Timed cortisol responses to insulin-hypoglycemia (IH) and arginine-vasopressin (AVP) were compared in 16 patients with pituitary tumors and six healthy controls. Serum cortisol was estimated by a specific and sensitive radioimmunoassay as per the WHO protocol. The basal cortisol (AM) was normal (> 290 nmol/l) in 9 patients and low in seven. With IH peak cortisol response was normal (> 550 nmol/l) in 10 patients, 8 of whom had normal and 2 had low basal (AM) cortisol. In contrast AVP evoked normal cortisol responses in only 5 of these patients, all of whom had normal basal (AM) cortisol and none had low basal (AM) cortisol. The data indicate lower sensitivity for AVP stimulation test (50%) and favour IH as the standard cortisol stimulation test in patients with pituitary tumors awaiting surgery.

We compared cortisol responses to 1 microgram adrenocorticotropic hormone (ACTH), 250 micrograms ACTH and insulin-induced hypoglycaemia (IIH), in patients suspected to have secondary hypocortisolism. Twenty-four patients (16 with hypothalamopituitary disorders and 8 on long-term glucocorticoid therapy) and eight healthy controls, underwent all three test protocols, with intervals of one day between each test. Mean cortisol responses to all three tests were comparable in both groups, but were more closely correlated for IIH vs. the 1 microgram ACTH test (r = 0.96) than for IIH vs. the 250 micrograms ACTH test (r = 0.88). Seven patients had discrepant results; all had a normal peak cortisol response to 250 micrograms ACTH (> 550 nmol/l), but a subnormal response to 1 microgram ACTH. Six of these also had a subnormal response to IIH. Cortisol responses to IIH match more closely those for 1 microgram ACTH in individual instances than those for 250 micrograms ACTH. The standard 250 micrograms ACTH stimulation, being supraphysiological, leads to underdiagnosis of the hypocortisolaemic state. The 1 microgram ACTH stimulation test should replace the standard 250 micrograms ACTH stimulation test in assessing the hypothalamo-pituitary-adrenocortical axis in secondary hypocortisolism.