DEFINITION — Polycystic ovary syndrome (PCOS) is a condition that causes irregular menstrual periods and elevated levels of androgens (male hormones) in women. The elevated androgen levels can sometimes cause excessive facial hair growth, acne, and/or male-pattern hair thinning. The condition occurs in about 5 to 10 percent of women.

Although PCOS is not completely reversible, there are a number of treatments that can reduce or minimize bothersome symptoms. Most women with PCOS are able to lead a normal life without significant complications.

CAUSE — The cause of PCOS is not completely understood. It is believed that abnormal levels of the pituitary hormone LH and high levels of male hormones (androgens) interfere with normal function of the ovaries. To explain how these hormones cause symptoms, it is helpful to understand the normal menstrual cycle.

Normal menstrual cycle — The brain (including the pituitary gland), ovaries, and uterus normally follow a sequence of events once per month; this sequence helps to prepare the body for pregnancy. Two hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH), are made by the pituitary gland. Two other hormones, progesterone and estrogen, are made by the ovaries.

During the first half of the cycle, small increases in FSH stimulate the ovary to develop a follicle (cyst) that contains an egg (oocyte). The follicle produces rising levels of estrogen, which cause the lining of the uterus to thicken and the pituitary to release a very large amount of LH. This midcycle "surge" of LH causes the egg to be released from the ovary (called ovulation, show figure 1). After ovulation, the ovary produces both estrogen and progesterone, which prepare the uterus for possible implantation and pregnancy.

Menstrual cycle in PCOS — In women with PCOS, multiple follicles (cysts) may develop. The follicles are unable to grow to a size that would trigger ovulation. Therefore, small follicles (4 to 9 mm in diameter) accumulate in the ovary, hence the term polycystic ovaries. None of these small follicles are capable of triggering ovulation. As a result, the levels of estrogen, progesterone, LH, and FSH become imbalanced.

Androgens (male-type hormones) are normally produced by the ovaries, the adrenal gland, and other sources. Examples of androgens include testosterone, androstenedione, dehydroepiandrosterone (DHEA), and DHEA sulfate (DHEA-S). Androgens may become increased in women with PCOS because of the high levels of LH, but also because of high levels of insulin that are usually seen with PCOS (see "Insulin abnormalities" under Signs and Symptoms).