Comparison of adrenocorticotropin (ACTH) stimulation tests and insulin hypoglycemia in normal humans: Low dose, standard high dose, and 8-hour ACTH-(1-24) infusion tests

The efficacy of the standard high dose ACTH stimulation test (HDT), using a pharmacological 250-mu g dose of synthetic ACTH-(1-24), in the diagnosis o f central hypoadrenalism is controversial. The insulin hypoglycemia test is widely regarded as the gold standard dynamic stimulation test of the hypot halamo-pituitary-adrenal (HPA) axis that provides the most reliable assessm ent of HPA axis integrity and reserve. Alternatively, a prolonged infusion of ACTH causes a continuing rise in plasma cortisol levels that may predict the adrenals' capacity to respond to severe ongoing stress. In nine normal subjects, we compared plasma ACTH and cortisol levels produc ed by three iv bolus low doses of ACTH-(1-24) (0.1, 0.5, and 1.0 mu g/1.73 m(2); LDTs) with those stimulated by hypoglycemia (0.15 U/kg insulin) and w ith the cortisol response to a standard 250-mu g dose of ACTH-(1-24). The n ormal cortisol response to an 8-h ACTH(1-24) infusion (250 mu g at a consta nt rate over 8 h) was determined using three modern cortisol assays: a high pressure liquid chromatography method (HPLC), a fluorescence polarization immunoassay (FPIA), and a standard RIA. In the LDTs, stepwise increases in mean peak plasma ACTH were observed (12. 4 +/- 2.0, 48.2 +/- 7.2, 120.2 +/- 15.5 pmol/L for the 0.1-, 0.5-, and 1.0- mu g LDTs, respectively; P values all <0.0022 when comparing peak values be tween tests). The peak plasma ACTH level after insulin-induced hypoglycemia was significantly lower than that produced in the 1.0-mu g LDT (69.6 +/- 9 .3 us. 120.2 +/- 15.5 pmol/L; P < 0.0002), but was higher than that obtaine d during the 0.5-mu g LDT (69.6 +/- 9.3 us. 48.2 +/- 7.2 pmol/L; P < 0.02). In the LDTs, statistically different, dose-dependent increases in peak cor tisol concentration occurred (355 +/- 16, 432 +/- 13, and 482 +/- 23 nmol/L ; greatest P value is is 0.0283 for comparisons between all tests). The pea k cortisol levels achieved during the LDTs were very different from those d uring the HDT (mean peak cortisol, 580 +/- 27 nmol/L; all P values <0.00009 . However, the mean 30 min response in the 1.0-mu g LDT did not differ from that in the HDT(471 +/- 22 us. 492 +/- 22 nmol/L; P = 0.2). In the 8-h ACT H infusion test, plasma cortisol concentrations progressively increased, re aching peak levels much higher than those in the HDT [995 +/- 50 vs. 580 +/ - 27 nmol/L (HPLC) and 1326 +/- 100 vs 759 +/- 31 nmol/L (FPLA)]. Significa nt differences in the basal, 1 h, and peak cortisol levels as determined by the three different assay methods (HPLC, FPIA, and RIA) were observed in t he 8-h infusion tests. Similarly, in the HDTs there were significant differ ences in the mean 30 and 60 min cortisol levels as measured by HPLC compare d with those determined by FPIA. We conclude that up to 30 min postinjection, 1.0 mu g/1.73 m(2) ACTH-(1-24) stimulates maximal adrenocortical secretion. Similar lower normal limits a t 30 min may be applied in the 1.0-mu g LDT and the HDT, but not when lower doses of ACTH-(1-24) are administered. The peak plasma ACTH level produced in the 1.0-mu g LDT is higher than in the insulin hypoglycemia test, but i s of the same order of magnitude. The peak cortisol concentration obtained during an 8-h synthetic ACTH-(1-24) infusion is considerably higher than th at stimulated by a standard bolus 250-mu g dose, potentially providing a me ans of evaluating the adrenocortical capacity to maintain maximal cortisol secretion. Appropriate interpretation of any of these tests of HPA axis fun ction relies on the accurate determination of normal response ranges, which may vary significantly depending on the cortisol assay used.