Cushing's syndrome - Causes of Cushing's syndrome

Most cases of Cushing's syndrome are related to the use of corticosteroid medication.

This is known as Iatogenic Cushing's syndrome.

Corticosteroids are used to:

• reduce inflammation in the body – this can be useful for treating conditions such as asthma or atopic eczema
• suppress the immune system – in conditions such as rheumatoid arthritis and lupus 

Corticosteroids are available in a number of different forms, including tablets (oral corticosteroids), sprays and inhalers (inhaled corticosteroids), creams and lotions (topical corticosteroids) and injections.

To help prevent side effects, corticosteroids are usually prescribed at the lowest possible effective dose. However, in people with severe symptoms that fail to respond to other forms of treatment, the only effective alternative is to prescribe a long-term course of high-dose corticosteroids.

Corticosteroids contain a synthetic (man-made) version of the cortisol hormone. Prolonged use can lead to cortisol levels building up and triggering Cushing’s syndrome. Misusing corticosteroids, or taking more than the recommended dose, also increases your risk of developing Cushing’s syndrome.

The risk of developing Cushing’s syndrome is higher in people who take oral corticosteroids (tablets), but it can also affect those who misuse inhaled or topical corticosteroids.

Endogenous Cushing’s syndrome

A much rarer type of Cushing's syndrome, known as endogenous Cushing's syndrome is caused by the body producing more cortisol than it needs.

The most common reason for this is a tumour (an abnormal growth of cells) which develops in the pituitary gland.

The pituitary gland is a pea-sized gland in the brain. It produces a hormone called adrenocorticotropin hormone (ACTH) which stimulates the adrenal glands, which in turn release cortisol into the blood.

The tumour can disrupt the normal workings of your pituitary gland so that it produces excessive amounts of ACTH, this causes the adrenal glands to produce too much cortisol.

Less commonly, a tumour develops inside one of the adrenal glands, leading to endogenous Cushing’s syndrome.

Another less common cause, is a tumour developing inside the lungs and producing the ATCH hormone, known as ectopic ACTH syndrome. This type of Cushing’s syndrome is more common in men.

Tumours that develop inside the pituitary or adrenal gland are usually benign (non-cancerous). Other than the symptoms of Cushing’s syndrome, they do not usually pose a serious threat to a person's health. Tumours that develop inside the lung can sometimes be cancerous. However, it is unclear why these tumours develop.