Mannitol refers to the levonorgestrel IUD system. An endometrial lining of 12mm is usually not normal after being put on a levonorgestrel IUD. Levonorgestrel IUD is a highly effective progestogen-like drug, after being put on levonorgestrel IUD can inhibit the growth of the uterine lining by releasing hormones into the uterine cavity, at this time the uterine lining is relatively thick at 12mm, which is not normal, and need to consult a doctor in time, and diagnostic curettage if necessary. Diagnostic curettage can find out whether there is endometrial overgrowth and endometrial pathology, if it is just simple endometrial growth will not have a greater impact on the body. If endometriosis is present, it is recommended that surgery be performed as soon as possible to remove the lesions to avoid life-threatening conditions.