Evaluation of low dose ACTH stimulation test in suspected Secondary Adrenocortical Insufficiency

BACKGROUND: Several studies in the last few years have shown that the standard 250µg dose used in ACTH stimulation test may be very high and have suggested that a dose of 1µg may be sufficient for evaluating hypothalamo-pituitary adrenal (HPA) axis. AIMS: To evaluate the role of low dose ACTH stimulation test in patients with suspected Secondary Adrenal Insufficiency (SAI). SUBJECTS AND METHODS: Thirty-one patients of suspected SAI with a documented pituitary lesion and seven normal healthy controls were included in the study. All the subjects underwent ACTH stimulation test with standard high dose (HD= 250 µg) and low dose (LD= 1µg) ACTH. Insulin Induced Hypoglycaemia (IIH) test was done in 7 out of 9 patients in whom the results of the two tests were discordant. The cut off for normal HD stimulation test was taken as peak cortisol response > 18 µg/dl and for LD test, either a cortisol response of > 18 µg/dl or an increment of more than 7 µg/dl over the basal value at any time during the test, on the basis of response observed in controls. RESULTS: 22/31 patients (74%) in SAI group showed concordant results with both tests, whereas nine patients had discordant results. These nine patients showed AI with LD ACTH, but HD ACTH test showed a normal response. In 7 of these 9 patients who underwent IIH, AI was confirmed in 6. CONCLUSION: The LD ACTH stimulation test seems to be better than HD ACTH stimulation test for evaluating HPA axis in patients with suspected SAI. When basal cortisol is normal, LD ACTH stimulation test detects subtle SAI