Researchers in Germany have found that a hormone called ACTH does not work properly in people with the condition.

ACTH plays an important role in helping the body cope with physical and psychological stress.

Like most diseases chronic fatigue syndrome is not triggered by a single factor

Dr Trudie Chalder, Institute of Psychiatry

The study is the latest attempt to discover the causes of this debilitating condition, which affects 243,000 people in the UK.

Previous studies have suggested that exposure to chemicals or allergies may trigger the syndrome.

Other studies have indicated that it may be caused by an abnormal immune system response to stress.

Stress tests

Dr Jens Gaab and colleagues at the University of Trier examined the effects of physical and psychological stress on 40 people, half of whom had been diagnosed with chronic fatigue syndrome.

The participants were asked to prepare for a fake job interview and to complete an arithmetic problem while under the impression they were being videotaped.

The second test measured physical stress on a stationary bicycle.

They participants also received an injection of insulin. This enabled the researchers to test hormonal reactions to stress.

The researchers found that people who had been diagnosed with chronic fatigue syndrome had lower levels of ACTH both before and after each of these tests.

The researchers said further study is needed to determine whether this hormone is responsible for the condition.

ACTH is one of the hormones released by the hypothalamus-pituitary-adrenal or HPA axis. This axis manages the body's response to stressful situations.

Dr Trudie Chalder, a senior lecturer at the Institute of Psychiatry in London, said many people studying chronic fatigue syndrome were carrying out research in this area.

"A number of people are looking at the HPA axis."

But she added: "Like most diseases chronic fatigue syndrome is not triggered by a single factor but rather a number of different factors."

The main feature of chronic fatigue syndrome is overwhelming tiredness, which disrupts the ability to carry out normal activities.

A number of other symptoms are commonly found such as sensitivity to light, dizziness, panic attacks, irritable bowel symptoms, menstrual problems, rashes, unexplained weight loss or gain and fever.

It substantially affects the ability to live a normal life.

The study is published in the journal Psychosomatic Medicine.