The exact cause of polycystic ovarian syndrome is unknown, but the condition tends to run in families.

Insulin resistance appears to be an important factor, particularly in women who are overweight. Many women with PCOS are resistant to insulin that is released by the pancreas to control sugars in the bloodstream. Higher levels of insulin are needed to act on the sugars in the fat and muscle cells, but the increased insulin levels cause the ovaries to produce too much testosterone. An excess of insulin and testosterone interferes with the development of the follicles, leading to a lack of ovulation.

Many women with PCOS are found to have raised levels of Luteinising Hormone (LH), which is created in the pituitary gland to stimulate ovulation and hormone production in the ovaries. This excess is thought to have an abnormal effect on the ovaries.

Being overweight makes the symptoms of PCOS more pronounced, as excess fat causes the body to produce more insulin. However, insulin resistance is itself a significant factor in polycystic ovarian syndrome, as raised insulin levels can lead to weight gain. Therefore, PCOS may be the cause of the obesity rather than obesity causing the condition itself.