To help you further understand how negative feedback regulation must be considered when diagnosing endocrine disorders, I have diagrammed how hormone secretion changes. These figures apply to the disorders shown in the table on the handout.
In the diagnosis of Cushing's Syndrome, one must first establish
that there is hypercortisolism. Cortisol secretion varies
extensively during the day, but there is a roughly circadian
pattern in which cortisol secretion is at its lowest around
midnight, thus cortisol levels are measured at this time.
("Circadian" means "about a day".) The level of ACTH distinguishes between
primary hypercortisolism and Cushing's
disease due to a pituitary adenoma.
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Primary adrenal insufficiency is referred to as Addison's disease. Addison's
disease usually has an autoimmune cause, but can also result from
tuberculosis. Note that the damage in Addison's disease will
affect both the zona glomerulosa and the zona fasciculata, so
there is hyposecretion of both cortisol
and aldosterone. While it
is true that decreased negative feedback will cause increased ACTH
secretion, in practice physicians will actually use a challenge
test to make the diagnosis. The challenge test determines
the ability of the adrenal gland to respond to exogenously
administered ACTH with increased cortisol secretion.
Secondary adrenal insufficiency is known by the appropriately descriptive term hypopituitary adrenal insufficiency. This disorder can result as a consequence of medical therapy with glucocorticoids, as diagrammed below.
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For further information regarding disorders of cortisol secretion (and a quiz), refer to the Negative Feedback Regulation page.