Cushing Syndrome II: Pathophysiology

The HPA axis maintains cortisol balance through a cascade: the hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary to secrete adrenocorticotropic hormone (ACTH). ACTH signals the zona fasciculata of the adrenal cortex to release cortisol. Rising cortisol levels suppress CRH and ACTH through negative feedback, maintaining homeostasis and preventing excess hormone production.

Long-term corticosteroid use suppresses CRH and ACTH through negative feedback, but continues to exert cortisol-like physiological effects despite low endogenous ACTH levels. Tissues remain exposed to high glucocorticoid activity, producing the same metabolic and immunologic consequences as endogenous forms. This disruption of normal feedback mechanisms distinguishes exogenous from endogenous Cushing syndrome.

In Cushing's disease, a benign anterior pituitary adenoma secretes excess ACTH autonomously and resists suppression by rising cortisol levels. This produces persistently high ACTH that chronically overstimulates the adrenal cortex, leading to elevated cortisol and increased adrenal androgen production. The tumor's resistance to feedback inhibition drives continuous excessive hormone secretion.

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. The tumor operates outside normal regulatory mechanisms.

In endogenous Cushing syndrome, the feedback loop is overridden by autonomous hormone production from either a pituitary adenoma or ectopic tumor. These sources resist suppression by elevated cortisol, allowing continuous excessive ACTH or cortisol secretion despite high circulating levels. The tumor's independence from normal regulatory signals prevents feedback inhibition from functioning properly.

Persistent cortisol elevation alters metabolism, immune function, and endocrine signaling, producing characteristic clinical features of Cushing syndrome. These metabolic and immunologic changes occur regardless of whether excess cortisol originates from external medications or internal pathology. The prolonged hormonal imbalance disrupts multiple physiological systems throughout the body.

The zona fasciculata of the adrenal cortex is the primary site where ACTH stimulates cortisol synthesis and release. In Cushing syndrome, chronic ACTH overstimulation of this region leads to excessive cortisol production and increased adrenal androgen secretion. This tissue responds to both normal physiological ACTH levels and pathologically elevated ACTH from tumors.