Is the plasma ACTH concentration a reliable parameter in the insulin tolerance test?
OBJECTIVE: The insulin tolerance test (ITT) is an established standardized test for the evaluation of the hypothalamic-pituitary-adrenal axis. While a peak cortisol value of >18 microg/dl is usually interpreted as a sufficient response to the ITT, the plasma ACTH response has not yet been standardized. METHODS: We evaluated retrospectively the peak plasma ACTH concentrations during 140 ITTs in 125 patients with suspected pituitary insufficiency and prospectively in 15 healthy subjects. RESULTS: All healthy subjects had a peak cortisol concentration >/=18 microg/dl; 32 of 125 tests in the patients showed an insufficient cortisol response (peak cortisol concentration <18 microg/dl). The peak stimulated ACTH concentration in patients with secondary adrenal insufficiency (SAI) was 49.2+/-37.2 pg/ml (mean+/-s.d.) vs 130.9+/-89.3 pg/ml in patients without SAI, and 110.9+/-55.4 pg/ml in normal subjects (P<0.001). There was a weak, but significantly positive correlation between the peak ACTH a
