Limited utility of oral glucose tolerance test in biochemically active acromegaly

Context: Measurement of GH after oral glucose tolerance test (OGTT) is used for the diagnosis and surveillance of acromegaly. However, there are major discrepancies between glucose-suppressed GH and plasma IGF1 as indices of biochemical activity of acromegaly in patients with relatively mild GH oversecretion. This study was aimed to assess the performance of OGTT in patients with acromegaly and variable GH outputs. Methods: Forty adults with newly diagnosed, untreated acromegaly (15 with GH > 4.3 mu g/l and 25 with GH < 4.3 mu g/l) and elevated IGF1 levels were studied. All underwent Q10 min for 24 h sampling for GH followed by an OGTT. Results: Postglucose nadir GH (GHn) correlated significantly to 24 h GHn, mean 24 h GH, and baseline GH (P < 0.001 for all comparisons). GHn correlated significantly to IGF1 z-scores for the 'low' GH group and for the entire group (P < 0.0001 for both comparisons), but not for the 'high' GH group. None of the patients with mean GH > 4.3 mu g/l had GHn below 1 mu g/l. In contrast, 13 out of 25 patients (52%) with GH < 4.3 mu g/l showed GHn lower than 1 mu g/l, and 7 of them (28%) had GHn lower than 0.4 mu g/l. These groups did not differ significantly either for average or for maximal GH suppression in OGTT. Conclusions: Our data show that suppressibility of GH by glucose in acromegaly is a function of the degree of GH hypersecretion and that OGTT has only limited diagnostic value in patients with biochemically active acromegaly but only mildly increased GH output.