The effect of metformin on prolactin levels in patients with drug-induced hyperprolactinemia

• Metformin reduces elevated, but not normal, prolactin levels.
• Prolactin-lowering effect of metformin is dose-dependent.
• This effect correlates with baseline prolactin levels.
• Metformin may bring benefits to patients with drug-induced hyperprolactinemia.

BackgroundIn bromocriptine-treated hyperprolactinemic patients with impaired glucose tolerance, metformin was found to reduce plasma levels of prolactin. No previous study has investigated its impact on plasma prolactin in patients with drug-induced hyperprolactinemia.MethodsThe study included 20 women with antipsychotic-induced hyperprolactinemia and 12 normoprolactinemic women, who, because of coexisting glucose metabolism abnormalities, were treated for 6 months with metformin. Hyperprolactinemic patients with prediabetes received moderate doses of metformin (1.7 g daily), while hyperprolactinemic and normoprolactinemic patients with type 2 diabetes were treated with high-dose metformin (2.55–3 g daily). Fasting plasma glucose levels, the homeostatic model assessment 1 of insulin resistance ratio (HOMA1-IR), glycated hemoglobin, as well as plasma levels of prolactin, thyrotropin, adrenocorticotropic hormone and insulin-like growth factor-1 were assessed at baseline and after 6 months of treatment.ResultsDespite reducing plasma glucose, HOMA1-IR, and glycated hemoglobin in all treatment groups, metformin decreased prolactin levels only if given at high doses to patients with elevated prolactin levels. No changes in thyrotropin, adrenocorticotropic hormone, and insulin-like growth factor-1 were observed in any treatment groups.ConclusionsThe obtained results suggest that the effect of metformin on plasma prolactin depends on its dose and is observed only in patients with elevated levels of this hormone.