The Utility of Oral Glucose Tolerance Testing for Diagnosis and Assessment of Treatment Outcomes in 166 Patients with Acromegaly

Context: GH suppression after oral glucose load [oral glucose tolerance test (OGTT)] and normal age- and gender-matched IGF-I levels reflect biochemical control of acromegaly. The OGTT is the gold standard for determining control of GH secretion at diagnosis and after surgical treatment, but the usefulness of performing an OGTT in patients treated with medical therapy has not been determined. Objective: Our objective was to assess relationships between basal GH levels (basal GH), GH responses to OGTT [GH nadir (GHn)], and IGF-I levels. Design: This was a retrospective electronic database review. Setting: This study was performed at a tertiary outpatient pituitary center. Patients: A total of 166 patients with acromegaly (79 females, 87 males) were included in the study. Four categories of testing were performed: diagnosis, postoperative assessment without medication, testing during somatostatin analog (SA) therapy, and testing during dopamine agonist (DA) therapy. Main Outcome Measures: Basal serum GH and IGF-I levels and GH levels 2 h after 75 g OGTT were measured. Results: A total of 482 simultaneous OGTT and IGF-I measurements were observed from 1985–2008. Discordant results of oral glucose tolerance testing (GHn and IGF-I) were observed 33, 48, and 18% in postoperative assessment without medication, SA, and DA categories, respectively. In the SA category, 42% of tests were discordant with normal IGF-I and nonsuppressed GHn. In contrast, 4% of tests were discordant with normal IGF-I and nonsuppressed GH in those treated with DA. No significant differences in discordance were observed when basal GH was used. Conclusions: Both basal and GHn levels are highly discordant with IGF-I levels during medical therapy with SAs. The OGTT is not useful in assessing biochemical control in these subjects.