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MCAT

Unit 2: Lesson 3

Foundation 3: Organ Systems

Cushing's syndrome and the hypothalamic-pituitary axis

Problem

Cushing syndrome, or hypercortisolism, results from abnormally high levels of the hormone cortisol circulating in the body. The causes of Cushing syndrome can be exogenous - for instance, due to people taking cortisol-like medications such as prednisone - or endogenous. Endogenous Cushing syndrome is less common and is most often caused by hormone-secreting tumors of the adrenal or the pituitary glands. Cushing Disease is a form of Cushing syndrome in which hypercortisolism results from the overproduction of adrenocorticotropic hormone (ACTH), caused by either a pituitary adenoma or a non-pituitary tumor.
Under normal conditions, cortisol regulates the production of corticotropin releasing hormone (CRH) from the hypothalamus and ACTH from the anterior pituitary, via negative feedback inhibition. In individuals with Cushing syndrome, abnormalities in the hypothalamic-pituitary axis (HPA) can be detected in the blood, and the specifics of these abnormalities can help determine the underlying etiology of Cushing syndrome.
Shown in table 1 are results from a study that examined the effects of Cushing’s syndrome on HPA hormones. CRH, ACTH, and cortisol levels were measured in rats injected with saline, cortisol, hormone X, or drug Y.
Table 1 Levels of hormones found in blood for three different treatments
Data adapted from: Cushing's Disease. UCLA Pituitary Tumor Program, Pituitary Tumor Surgery
Rats injected with hormone X would serve as the best animal model for which of the following?
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