Addison's disease is caused when your adrenal cortex (the outer layer of your adrenal glands) is damaged. The most common reason for this is an autoimmune condition.

Autoimmune conditions

Your immune system is your body's defence against infection and disease. If you are ill, your immune system will start to produce antibodies (a special type of protein to destroy disease-carrying organisms and toxins). These antibodies will attack the cause of the illness.

If you have an autoimmune condition, such as diabetes, your immune system starts producing antibodies that attack your own tissues and organs.

These immune cells can cause Addison's disease if they start to destroy your adrenal cortex. If 90% of the adrenal cortex is destroyed, your adrenal glands will not be able to produce enough of the steroid hormones, cortisol and aldosterone. Once the levels of these start decreasing, you will start to experience the symptoms of Addison's disease.

In the developed world, autoimmune conditions cause around 80% of all cases of Addison's disease, which is sometimes called autoimmune Addison's disease. In the UK, there are currently around 8,400 diagnosed cases of autoimmune Addison's disease.

Genetics

Research has shown that if you have certain gene variants (units of genetic material that determine many of your body's characteristics), you may be more likely to have an autoimmune condition. A particular piece of DNA coding has been found in a number of people with autoimmune Addison's disease that is also present in people with other autoimmune disorders.

Exactly how this gene contributes to the development of Addison's disease is not yet clear, but it does enforce the link between Addison's disease and other autoimmune conditions. Therefore, if you have another autoimmune condition, you are slightly more likely to develop Addison's disease at some point.

Research has suggested that up to half of all people with autoimmune Addison's disease will have another autoimmune condition. Some of the more common autoimmune conditions that have been linked to Addison's disease are:

• vitiligo: a chronic (long-term) condition that causes pale, white patches to develop on the skin
• type 1 diabetes: a chronic condition that is caused by too much glucose in the blood  
• hypothyroidism: an underactive thyroid gland

Other causes

Tuberculosis (TB) used to be a major health problem in England, and it still is in some parts of the developing world. TB is a bacterial infection that mostly affects the lungs but it can also spread to other parts of your body. TB can cause Addison's disease if it destroys your adrenal glands.

Some other possible causes of Addison's disease include:

• infections: such as those that are linked to AIDS, or fungal infections
• a haemorrhage: very heavy bleeding into the adrenal glands, sometimes associated with meningitis or other severe sepsis
• cancer: if cancer cells from elsewhere in your body spread to your adrenal glands
• amyloidosis: a disease where amyloid, a protein that is produced by the cells in your bone marrow, builds up in your adrenal glands and damages them
• surgical removal of the adrenal glands (adrenalectomy), for example to remove a tumour
• a genetic defect with your adrenal glands which means that they do not develop properly or cannot produce hormones