Most cases of Cushing's syndrome are related to long-term use of corticosteroid medication.
This is known as iatrogenic Cushing's syndrome.
Corticosteroids are used to:
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 corticosteroids tablets, but it can also affect those who misuse inhalers or corticosteroid creams.
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) that develops in the pituitary gland.
The pituitary gland is a pea-sized gland in the brain. It produces a hormone called adrenocorticotropin hormone (ACTH) that stimulates the adrenal glands, which in turn release cortisol into the blood.
The tumour can disrupt the normal workings of your pituitary gland so 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, which leads to Cushing's syndrome. Another less common cause is a tumour developing inside the lungs and producing the ACTH hormone, known as ectopic ACTH syndrome.
Tumours that develop inside the pituitary or adrenal gland are usually benign (non-cancerous). Other than the symptoms of Cushing's syndrome, they don't usually pose a serious threat to a person's health. Tumours that develop inside the lung can sometimes be cancerous. It's unclear why these tumours develop.