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Course: MCAT > Unit 2

Lesson 3: Foundation 3: Organ Systems

Endocrine: Cushing syndrome and the HPA axis


Cushing syndrome, or hypercortisolism, results from abnormally high levels of the glucocorticoid hormone cortisol circulating in the body. The symptoms of Cushing syndrome can vary between individuals, but the classic signs in humans include rounded face, upper body obesity, fat deposits in the neck, and thin arms and legs.
The causes of Cushing syndrome can be exogenous - for instance, due to people taking cortisol-like medications such as prednisone - or endogenous, having an internal cause or origin. Endogenous Cushing syndrome is less common and is most often caused by hormone-secreting tumors of the adrenal or the pituitary glands. Cushing disease, for instance, 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, levels of hormones involved in the hypothalamic-pituitary-adrenal (HPA) axis can be measured in the blood, and the results of these measurements can help determine the underlying etiology of Cushing syndrome.
Table 1 shows data from a study investigating abnormalities in the HPA axis. In this study, 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
What is the function of the rats injected with saline?
Choose 1 answer: