Comparison of Glucagon Stimulation Test and Insulin Tolerance Test for Assessment of an Anterior Pituitary Function-A...

Background: The insulin tolerance test (ITT) and glucagon stimulation test (GST) are commonly used for assessment of anterior pituitary function, but there are few direct comparisons of these tests, i.e., where both tests were performed on the same individuals. Methods: We designed a cross-over study, where we compared concentrations of glucose, cortisol, and GH during ITT and standard fixed-dose GST in 19 subjects (five males), with a mean age of 33.8 years (range 19-60) and a mean BMI of 27.8 kg/m² (range 16.5-47.6). Results: Optimal fall in glucose concentrations during ITT (i.e., <40 mg/dL) was obtained in all study participants. During ITT we obtained lower minimal glucose concentrations (glucose nadir), i.e., 29.7 ± 7.67 mg/dL at 30 min of ITT, than during GST, i.e., 73.6 ± 9.67 mg/dL at 180 min of GST, p < 0.01. In contrast, glucose fluctuations (ΔGlucose) were higher during GST (77.8 ± 22.6 mg/dL versus 56.7 ± 10.9 mg/dL, p = 0.002, for GST and ITT, respectively). There was, however, no difference in degree of stimulation of either cortisol or GH release during both tests: ΔCortisol 9.28 ± 3.79 µg/dL for GST versus 8.49 ± 3.46 µg/dL for ITT, p = 0.4, and ΔGH 10.23 ± 10.36 ng/mL for GST versus 10.52 ± 9.67 ng/mL for ITT, p = 1.0. Conclusions: Although hypoglycaemia is not observed during GST in contrast to ITT, it appears that both tests lead to similar increments in cortisol and growth hormone secretion. We, therefore, conclude that GST should not be automatically considered as an "inferior" option in comparison to ITT.