Metformin was found to decrease elevated levels of anterior pituitary hormones. Its impact on lactotrope secretory function was absent in women with vitamin D insufficiency. This study investigated whether vitamin D status determines metformin action on overactive gonadotropes. We compared the effect of six-month metformin treatment on the plasma levels of gonadotropins, TSH, prolactin, ACTH, estradiol, free thyroid hormones, IGF-1, and 25-hydroxyvitamin D, as well as on glucose homeostasis markers between three matched groups of postmenopausal women at high risk for diabetes: untreated subjects with vitamin D insufficiency (group A), untreated women with normal vitamin D status (group B), and individuals receiving vitamin D supplementation with normal 25-hydroxyvitamin D levels (group C). Only in groups B and C did metformin reduce FSH levels and tend to decrease LH levels, and these effects correlated with baseline gonadotropin levels, baseline 25-hydroxyvitamin D levels, and the improvement in insulin sensitivity. Follow-up gonadotropin levels were higher in group A than in the other two groups. The drug did not affect circulating levels of TSH, prolactin, ACTH, estradiol, free thyroid hormones, IGF-1, or 25-hydroxyvitamin D. The obtained results suggest that the impact of metformin on gonadotropin secretion in women after menopause is determined by vitamin D status.