Addison's disease develops when the outer layer of your adrenal glands (your adrenal cortex) is damaged, reducing the levels of hormones it produces.

Problems with the immune system

In the UK, a problem with the immune system is the most common cause of Addison's disease, accounting for 70-90% of cases.

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

However, if you develop a problem with your immune system, it can start to attack your own healthy tissues and organs. This is known as an autoimmune disorder.

Addison’s disease can develop if your immune system attacks your adrenal glands and severely damages your adrenal cortex. When 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 levels of these start decreasing, you will experience symptoms of Addison’s disease.

It's not clear why some people develop this problem with their immune system, although it can run in families (see below).

Genetics

Research has shown some people with certain genes are more likely to develop autoimmune disorders.

It is not clear how these genes lead to Addison's disease and similar conditions, but it does mean your risk of developing Addison's disease is increased if you or a close family member have another autoimmune condition, such as:

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

Other causes

Tuberculosis (TB) is the most common cause of Addison's disease worldwide, but is rare in the UK. TB is a bacterial infection that mostly affects the lungs but can also spread to other parts of your body. It can cause Addison’s disease if it damages your adrenal glands.

Other possible causes of Addison’s disease include:

• infections - such as those linked to AIDS, or fungal infections
• a haemorrhage - very heavy bleeding into the adrenal glands, sometimes associated with meningitis or other types of severe sepsis
• cancer - if cancer cells from elsewhere in your body spread to your adrenal glands
• amyloidosis - a disease where amyloid, a protein produced by your bone marrow cells, builds up in your adrenal glands and damages them
• surgical removal of the adrenal glands (adrenalectomy), for example to remove a tumour
• adrenoleukodystrophy (ALD) - a rare, life-limiting inherited condition affecting the adrenal glands and nerve cells in the brain that most often affects young boys