Myo-Inositol Potentiates the Inhibitory Effect of Metformin on Prolactin Levels.

INTRODUCTION: Metformin was found to reduce elevated levels of anterior pituitary hormones. Its thyrotropin-lowering effect was more pronounced in individuals receiving myo-inositol. The aim of the present study was to investigate whether the concomitant supplementation of myo-inositol determines the impact of metformin on prolactin levels.

METHODS: The study population consisted of two groups of women with mild-to-moderate hyperprolactinemia. Group 1 included 24 individuals receiving myo-inositol preparations (2 g daily for at least 6 months), while 24 inositol-naïve women belonged to group 2. Both groups were matched for age, insulin sensitivity, and prolactin concentration. For the following 6 months, all women were treated with metformin (1.7 daily). Plasma glucose levels, the homeostatic model assessment of insulin resistance ratio (HOMA-IR), glycated hemoglobin, as well as plasma levels of total prolactin, monomeric prolactin, thyrotropin, free thyroid hormones, adrenocorticotropic hormone, and insulin-like growth factor-1 were measured at baseline and after 6 months of metformin treatment.

RESULTS: Metformin reduced plasma glucose, HOMA-IR, and glycated hemoglobin in both study groups, but this effect was more pronounced in group 1 than group 2. Treatment-induced changes in total and monomeric prolactin levels were significant only in group 1. There were no differences between follow-up and baseline values of thyrotropin, free thyroxine, free tri-iodothyronine, adrenocorticotropic hormone, and insulin-like growth factor-1. Treatment-induced changes in prolactin concentration correlated with baseline prolactin levels, baseline values of HOMA-IR, and the impact of treatment on HOMA-IR.

DISCUSSION: The obtained results suggest that myo-inositol supplementation potentiates the inhibitory effect of metformin on prolactin levels in women with hyperprolactinemia.