Polycystic Ovarian Syndrome (PCOS) is an endocrine (hormonal) disorder, affecting between 5 and 10% of premenopausal women. It is characterised by polycystic ovaries (seen on ultrasound), hyperandrogenism (high levels of the male hormones testosterone and androstenedione) and anovulation (irregular or absent menstrual periods).

A large number of women (~20% in the reproductive age) have polycystic appearing ovaries (PCO) but not all of these have the associated problems which characterise PCOS.

Symptoms usually present during puberty or in the early to mid 20’s.

Exactly why PCOS occurs is still unclear, but we do know that there are a number of different causes. 

The most common cause appears to be insulin resistance, a condition that also leads to the development of type 2 diabetes.  In insulin resistance, the body's insulin cannot do it's job of getting glucose from the bloodstream (where it comes from the food we eat) into the muscles and cells properly.  So the body responds to this by producing more and more insulin, to try to overcome the 'resistance'.  In women with PCOS, high insulin levels act on the ovaries and adrenal glands to cause them to produce more male hormones.  Higher levels of male hormones upset the hormonal balance, preventing ovulation.

Other causes of PCOS include incomplete development of the fertility centre in the brain, damage to the fertility centre by eating disorders, and rare conditions including pituitary tumours and congenital adrenal hyperplasia, where excess male hormones are produced by the adrenal glands.

For a great explanation of PCOS and its causes download this article by Dr Warren Kidson and Dr James Talbot.