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Cortisol production is normally governed by the hypothalamic–pituitary–adrenal (HPA) axis, which maintains hormonal balance through tightly regulated feedback mechanisms. Disruption of this regulatory system is central to the development of Cushing syndrome, whether the excess cortisol originates from external medications or internal pathology. Persistent cortisol elevation alters metabolism, immune function, and endocrine signaling, producing the characteristic clinical features of the disorder.
Normal HPA Axis Regulation
Under typical physiological conditions, the hypothalamus secretes corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH). ACTH then acts on the zona fasciculata of the adrenal cortex, promoting cortisol synthesis and release. As circulating cortisol levels rise, they inhibit further CRH and ACTH secretion through negative feedback, maintaining homeostasis.
Feedback Disruption in Exogenous Cushing Syndrome
In medication-induced Cushing syndrome, prolonged exposure to glucocorticoids such as prednisone suppresses CRH and ACTH through negative feedback but continues to exert cortisol-like physiological effects. Despite low endogenous ACTH, tissues remain exposed to high glucocorticoid activity, leading to the same metabolic and immunologic consequences seen in endogenous forms of the syndrome.
Endogenous Overproduction of Cortisol
Endogenous Cushing syndrome arises when this feedback loop is overridden. In Cushing’s disease, a benign anterior pituitary adenoma secretes excess ACTH autonomously. The tumor is resistant to suppression by rising cortisol, producing persistently high ACTH levels that overstimulate the adrenal cortex. This leads to elevated cortisol and increased adrenal androgen production.
Ectopic ACTH Production
In ectopic ACTH syndrome, non-pituitary tumors—commonly located in the lungs or pancreas—synthesize ACTH independently of hypothalamic or pituitary control. This unregulated hormone secretion drives adrenal cortisol production upward, bypassing the usual feedback restraints of the HPA axis and resulting in marked hypercortisolism.
