MON-073 A Case of Acromegaly with Normal Response to Oral Glucose Tolerance Test

Abstract Disclosure: T. Vu: None. W. Yong: None. T.N. Othman: None. J.Q. Dai-ju: None. Background: Serum IGF-1 level is the typical first-line screen for acromegaly. If the IGF-1 level is elevated, an oral glucose tolerance test (OGTT) is utilized for confirmation. A GH nadir level greater than 1 ng/mL (measured every 30 minutes for 120 minutes after the administration of 75 g oral glucose) has been the standard threshold for diagnosing acromegaly. However, this test may not be sensitive enough in a proportion of the population. The following case highlights the importance of clinical suspicion for acromegaly in patients with persistently elevated IGF-1 but normal OGTT.Clinical Case: A 55-year-old female with history of type II diabetes mellitus and hyperprolactinemia treated with bromocriptine twenty years ago presented with persistently elevated prolactin (39.1 ng/mL) and IGF-1 (306 ng/mL). OGTT was repeated three times. GH was as low as 0.8 ng/mL at 30 minutes in the first test and 0.3 ng/mL in the other two. Interestingly, GH surged up (with 130-533% over the basal level) by the 60-minute mark in all three tests. Initial pituitary MRI revealed a 7x3 mm pituitary microadenoma with the infundibulum displaced to the right. MRI repeated three years later showed the adenoma had increased to a size of 10x4 mm. The decision was made for transsphenoidal surgery due to the increased size of the mass. Histopathologic evaluation revealed a tumor with pleomorphic nuclei, nucleoli and abundant eosinophilic cytoplasm that had diffuse immunopositivity for pituitary-specific, positive transcription factor 1 (PIT-1), and human GH in adenoma cells, supporting the diagnosis of a PIT-1 lineage pituitary somatotroph tumor. Repeat MRI has not shown evidence of disease recurrence and IGF-1 has been within normal range for age and gender nineteen months post-operation. Conclusion: For patients undergoing OGTT as a confirmatory test, a GH nadir level greater than 1 ng/mL may be too high and not sensitive enough for diagnosing acromegaly. A paradoxical GH response to OG load should be taken into consideration and raise clinical suspicion for acromegaly. References: Mukai K, Otsuki M, et al. Clinical Characteristics of Acromegalic Patients with Paradoxical GH Response to Oral Glucose Load. 2019. JCEM 104:1637-1644. Presentation: Monday, July 14, 2025