Myo-Inositol Enhances the Inhibitory Effect of Metformin on Gonadotropin Levels in Postmenopausal Women

Objectives: Metformin decreased circulating levels of anterior pituitary hormones and its effect on thyrotropin concentration was found to be stronger in individuals receiving myo-inositol. Phospholipids containing inositols are precursors of second messengers of several hormones, including gonadotropins and insulin. The aim of the current study was to investigate whether the concomitant use of myo-inositol changes the effect of metformin on gonadotropin levels. Design: A prospective observational study. Participants/Materials, Setting, and Methods: This study, conducted at a university-affiliated medical center, included two groups of postmenopausal women with prediabetes, matched for age, FSH and LH levels, and insulin sensitivity: women taking myo-inositol preparations for at least 6 months (group A, n = 23) and women not receiving inositol preparations (group B, n = 23). All participants were treated with metformin (850 mg twice daily) for the following 6 months. At the beginning and at the end of the study, we assessed plasma glucose, insulin, FSH, LH, thyrotropin, prolactin, adrenocorticotropic hormone, insulin-like growth factor-1, estradiol, and glycated hemoglobin. Results: The impact of metformin on glucose levels, the homeostatic model assessment 1 of insulin resistance ratio, and glycated hemoglobin was more pronounced in group A than in group B. Metformin administered with myo-inositol reduced both FSH and LH. No significant changes in gonadotropin levels were observed in women receiving only metformin. The impact on FSH and LH levels correlated with their baseline concentrations and with the degree of improvement in insulin sensitivity. Levels of the remaining hormones did not change throughout the study. Limitations: The most important limitation of the study is a relatively small number of participants. Moreover, the study protocol does not allow to conclude whether similar effects are observed in premenopausal women. Conclusions: Myo-inositol may enhance the inhibitory effect of metformin on gonadotropin production in postmenopausal women.