Endocrine Abstracts

Objective: Change in somatostatinergic tone (SST) is one of the key features of acromegaly, but the effect of tumor size on SST is not clear.

Design: The aim of this study was to determine how SST changes depending upon the size of GH-producing pituitary tumors.

Method: GH levels of 29 patients with newly diagnosed acromegaly were measured using a 75 g oral glucose tolerance test (OGTT), an insulin tolerance test (ITT) and an octreotide suppression test (OST). GH/glucose ratio during the OGTT and the ITT were calculated, and GH levels during the OGTT were subtracted from those during the ITT and the OST at the same time point (ΔGHIO and ΔGHSO respectively). These values were compared according to the size of tumors and the response pattern to the OST.

Results: Twenty-two of 29 tumors were macroadenomas. The ratio between GH and glucose (GH/glucose) during the OGTT but not the ITT was also higher in macroadenomas than microadenomas with borderline significance. ΔGHIO and ΔGHSO during the entire test time also were different between macroadenomas and microadenomas with borderline significance. According to the further analyses with macroadenomas based upon the response pattern to the OST, GH/glucose for both the OGTT and the ITT were higher in non-responders with borderline significance while ΔGHIO nor ΔGHSO did produce significant differences between responders and non-responders.

Conclusion: Our study suggests that, as the size of the tumor becomes large, the effect of glucose on SST was attenuated. Macroadenomas with non-responders to the OST had the portion of GH secretion beyond the range of regulation by SST. This is the first study to examine changes in SST of GH-producing pituitary tumors through the additional manipulation of test results necessary to confirm the diagnosis of acromegaly.

Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.

Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.