Endogenous testosterone determines metformin action on prolactin levels in hyperprolactinaemic men: A pilot study.

Metformin was found to reduce elevated prolactin levels in women but not in men. The current study was aimed at investigating whether endogenous testosterone determines the impact of metformin on lactotroph function in men. This prospective case-control study included 28 men with recently diagnosed type 2 diabetes mellitus and mild or moderate hyperprolactinaemia, 14 of whom had low testosterone levels, while in the remaining 14 ones' testosterone levels were within the reference range. All participants received metformin (2.55-3 g daily) for the following 4 months. Circulating levels of glucose, insulin, prolactin, testosterone, oestradiol, gonadotropins, sex hormone-binding globulin adrenocorticotropic hormone, insulin-like growth factor-1, thyrotropin and free thyroid hormones were measured at the beginning and at the end of the study. Although metformin reduced plasma glucose levels and improved insulin sensitivity in both groups, this effect was stronger in participants with low testosterone levels. Only in patients with abnormally low testosterone levels, the drug decreased prolactin levels. This effect, which was accompanied by an increase in luteinizing hormone levels, was inversely correlated with baseline testosterone and calculated free testosterone levels, and positively with treatment-induced improvement in insulin sensitivity. In both treatment groups, metformin produced a neutral effect on plasma levels of the remaining hormones. The obtained results suggest that endogenous testosterone may attenuate the impact of metformin on lactotropic cells.