Hormonal IUDs (brand names Mirena, Skyla, and Liletta; referred to as intrauterine systems in the UK) work by releasing a small amount of levonorgestrel, a progestin. The primary mechanism of action is making the inside of the uterus fatal to sperm.[29] Because they thin the endometrial lining, they reduce or even prevent menstrual bleeding, and can be used to treat menorrhagia (heavy menses), once pathologic causes of menorrhagia (such as uterine polyps) have been ruled out.[30]