Causes of PCOS

The exact cause is unknown, but it’s thought to be related to abnormal hormone levels, and is often associated with insulin resistance.

Resistance to insulin

Insulin is a hormone produced by the pancreas to control the amount of sugar in the blood. It works particularly on the liver and muscle cells, causing them to absorb more glucose from blood where it is either broken down to produce energy or converted to long term energy stores.

When someone is resistant to the effects of insulin, the blood sugar lowering action does not work properly and the body produces extra insulin.

Insulin has another effect; it causes the ovaries to produce testosterone. In most women this is within the normal range, but women with PCOS who have too much insulin may also produce too much testosterone. The raised levels of testosterone can give rise to some of the symptoms associated with PCOS, such as excess hair growth and head hair loss.

High levels of insulin and testosterone may also prevent the normal development of follicles in the ovaries, with many not developing fully. This causes problems with ovulation, so many women have period problems and reduced fertility.

Insulin resistance can also lead to weight gain. Excess fat causes the body to produce even more insulin which can make the symptoms worse. A vicious circle can result so it’s important to try to minimise weight gain.


PCOS sometimes runs in families. Women who have close relatives – e.g. mother, aunt or sister – with PCOS are also more likely to be affected. This suggests there may be a genetic link, although specific genes have not yet been identified.