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Causes - Polycystic ovary syndrome (PCOS)

The exact cause of polycystic ovary syndrome (PCOS) is unknown.

It is thought to be linked to abnormal hormone levels, a dysfunction of the ovaries and to run in families.

Two main problems thought to cause PCOS are:

  • resistance to insulin
  • hormone imbalance

Read about the symptoms of PCOS

Resistance to insulin

Insulin is a hormone produced by your body to help it control the amount of sugar in your blood (glucose). It helps to move glucose from your blood into the cells of your body, where it's broken down to produce energy.

Insulin resistance means your body's tissues are resistant to the effects of insulin. Because of this, your body has to produce extra insulin.

High levels of insulin cause the ovaries to produce too much testosterone. This interferes with the development of the follicles and prevents normal ovulation. The follicles are sacs in the ovaries where eggs develop.

Insulin resistance can also lead to weight gain. This can make PCOS symptoms worse. This is because having excess fat causes the body to produce even more insulin.

If you have PCOS and are overweight, the first step is to reach a normal weight.

Hormone imbalance

Many women with PCOS have an imbalance in certain hormones.

Raised levels of testosterone

Testosterone is often thought of as a male hormone. But all women usually produce small amounts of it. Women with PCOS may make more testosterone than usual. This causes many of the symptoms of PCOS.

Raised levels of luteinising hormone (LH)

This stimulates ovulation but may have an abnormal effect on the ovaries if levels are too high. It can also work with insulin and cause higher levels of male hormones.

Low levels of sex hormone-binding globulin (SHBG)

This is a protein in the blood. It binds to testosterone and reduces its effect. If your SHBG levels are low, it means there is more 'free testosterone' in your blood. The SHBG is consumed by the excess testosterone that it attaches to.


PCOS sometimes runs in families. If any relatives, such as your mother, sister or aunt have PCOS, you are at an increased risk of developing it.

Specific genes associated with the condition have not yet been identified. But research to understand this condition is progressing fast.

Content supplied by the NHS and adapted for Ireland by the HSE

Page last reviewed: 23 March 2021
Next review due: 23 March 2024

This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.