What is the most common cause of Cushing's syndrome?

The most common cause of Cushing's syndrome is prolonged corticosteroid use. Cushing's syndrome occurs when there is excessive cortisol in the body. While the body can produce cortisol naturally, external sources—primarily in the form of glucocorticoid medications—can lead to a condition where cortisol levels are elevated. Individuals receiving long-term treatment with corticosteroids for conditions such as asthma, rheumatoid arthritis, or other inflammatory disorders often develop symptoms of Cushing's syndrome due to this external source of cortisol.

Genetic predisposition, adrenal tumors, and hypothalamic dysfunction are less common causes. While an adrenal tumor can produce excess cortisol, and dysfunction in the hypothalamus can lead to excessive adrenocorticotropic hormone (ACTH) production, these scenarios are not as prevalent as the use of corticosteroids. The widespread use of corticosteroid medications in clinical settings significantly increases the incidence of iatrogenic Cushing's syndrome, making it the leading cause in most cases.