It's completely natural for a woman's body to produce a small amount of testosterone as well as another masculine sex hormone, androstenedione. The problem in PCOS is that the ovaries make too much luteinizing hormone (LH), which later turns into androstenedione and testosterone. LH is also the hormone that in excess traps the egg inside the ovary and interferes with ovulation.
Low-dose contraceptives reduce the production of this hormone. With less LH to trigger the release of testosterone, use of the Pill stops some of the symptoms of excess testosterone, such as acne and unusual hair growth. A longer-term benefit of the Pill in treating PCOS is preventing overgrowth of the lining of the uterus by restoring normal menstrual periods. All the various brands of the Pill contain a compound called progestin, almost always combined with some kind of estrogen. Progestin provides the uterus with progesterone.
Progesterone is the hormone that causes the uterus to thicken after ovulation so it can receive the fertilized egg. If the egg is not fertilized, the thickening of the endometrium of the uterus is sloughed off during the menstrual period and the whole cycle starts over again. PCOS is a condition in which there is not enough progesterone. The endometrium that lines the uterus never sloughs off or only partially sloughs off, so women with PCOS have scanty or no periods. The endometrium gets thicker and thicker and over a period of years there can be endometriosis or endometrial cancer.
Restoring normal periods is important to lowering the risk of these painful and even life-threatening complications of the disease. It's also possible for a woman who has PCOS to have no period at all one month, and then a heavy period the next. The progesterone provided by the Pill evens out the menstrual cycle so that the woman's periods eventually occur on time with normal volume. The complete benefits of taking oral contraceptives for PCOS are not immediate. Successful hormone treatment takes at least several months. It's important to take the Pill at the same time every day while hormone balance slowly improves.