Heavy menstrual flow can be caused by a thick endometrium, or by adenomyosis, submucosal fibroids, or endocrine disorders. Menstruation is the cyclical shedding of the uterine lining, and a thicker lining of the uterus will result in heavy menstrual flow, such as in patients with endometrial hyperplasia or endometrial polyps. Menopausal or adolescent women with endocrine disorders may also experience heavy periods. Adenomyosis, in which the endometrium grows into the myometrium, resulting in increased uterine size, restricted growth of the myometrium, and heavy menstruation with dysmenorrhea. Submucosal fibroids, fibroids grow under the endometrium, making the uterine cavity larger, increasing the area of the endometrium, and affecting the uterine contraction, which is prone to heavy menstrual flow, prolonged menstrual period and other symptoms. Therefore, heavy menstruation is not always caused by thick endometrium. If you find heavy menstruation, you need to consult a doctor.