The obtained correlations (positive between metformin action on plasma prolactin and baseline prolactin levels and negative between the impact on plasma prolactin and thyroid antibody titers) suggest, however, that metformin may reduce prolactin levels also in patients with severe hyperprolactinemia, but that this effect is probably also attenuated by coexisting autoimmune thyroiditis. The gene discussed is PRL; the disease is autoimmune thyroid disease.