Investigation of serum incretin levels in patients with prolactinoma

There have been conflicting results between prolactin and glucose metabolism. Hyperprolactinemia may cause insulin resistance and DM development with its possible effects on incretins. In our study, we aimed to evaluate whether the incretin system is healthy in patients with prolactinoma. Thirteen patients with prolactinoma without any history of diabetes and 13 healthy volunteers were included in the study. The oral glucose tolerance test was performed with 75 g glucose. Glucagon-like peptide 1 (GLP-1) and Glucose-dependent Insulinotropic Polypeptide (GIP) levels were measured at 0, 30, 60, 90 and 120th minutes. Basal glucose levels were similar in both groups (p> 0.05). Although glucose levels of the group with prolactinoma were higher in the follow-up after the 30th minute compared to the healthy group, there was no statistically significant difference in glucose levels between OGTT (30th, 60th, 90th and 120th minutes). (p> 0.05). However, when the groups were parallel to each other, it was statistically significant that there was no rapid decline after 30 minutes in the prolactinoma group compared to the group with healthy volunteers (p= 0.035). There was no statistically significant difference between the basal insulin levels and the insulin levels measured during OGTT (30th, 60th, 90th and 120th minutes) (p> 0.05). GLP-1 was found to be similar in prolactinoma group and healthy control group in all measurements (p> 0.05). The areas under the curve (AUC) are evaluated for GLP-1 measurements; there was no statistically significant difference in prolactinoma group compared to healthy control group (p= 0.088). GIP levels were evaluated between the two groups, the GIP levels measured during baseline and OGTT were significantly lower. There was statistically significant difference between baseline GIP levels (p= 0.024). There was statistically significant differences in GIP levels at 30th, 90th and 120th minutes during OGTT (p= 0.006, p= 0.015, p= 0.035). The areas under the curve (AUC) are evaluated for GIP levels; there was a statistically significant difference in prolactinoma group compared to healthy control group (p= 0.011). These findings suggest that the statistically significant GIP level between the two groups; the difference in GLP-1, glucose and insulin levels, although not significant, may suggest that incretins may play a role in the mechanism of increased insulin resistance in patients with prolactinoma. In order to show the possible relationship between glucose metabolism disorder and incretin effect in patients with prolactinoma and / or hyperprolactinemia, studies must be done in larger case series. Key Words: GLP-1, GIP, Incretin, Prolactinoma, Hyperprolactinemia