Below is a flow chart showing how the secretion of cortisol is controlled. This is a classic negative feedback system. Both the hypothalamus and anterior pituitary gland measure the amount of cortisol in the blood. To increase cortisol production in the adrenal gland, the hypothalamus releases the hormone GnRH. GnRH acts on the anterior pituitary to increase the release of the hormone ACTH. ACTH acts on the adrenal gland to increase the production of cortisol.
Failure to produce adequate amounts of cortisol causes Addison's disease which is a life-threatening condition. Addison's disease could be the result of problems in the hypothalamus, anterior pituitary or adrenal gland. Use the flowchart to explain how you would identify the likely site of dysfunction in a patient with Addison's disease.

The aim of the negative feedback system is to maintain appropriate cortisol levels in the blood. If cortisol levels are too low the hypothalamus should release GnRH in the anterior pituitary gland should release ACTH. If a person had hypoadrenocorticism (i.e. Addison's disease) because of a failure in the hypothalamus we would expect to see very low levels of GnRH. If the problem was in the anterior pituitary gland we would expect to see very low levels of ACTH, but GnRH levels would be high (if they could be measured). If a patient had a problem in the adrenal gland and we would expect to see that both GnRH and ACTH levels would be increased.