Adrenocorticotropic Hormone (ACTH, Corticotropin)

In response to a stimulus such as stress, the hypothalamus secretes corticotropinreleasing hormone. This hormone stimulates the secretion of adrenocorticotropic hormone (ACTH) by the anterior pituitary gland. ACTH, in turn, causes the adrenal cortex to release the glucocorticoid hormone cortisol. As levels of cortisol in the blood rise, the pituitary gland is stimulated to decrease ACTH production via a negative feedback mechanism. (See Appendix B for description of hormonal feedback process).
Diurnal variations in ACTH levels occur, with peak levels occurring between 6 and 8 AM and trough levels occurring between 6 and 11 PM. Trough levels are approximately one-half to two-thirds the peak levels. Assessment of ACTH levels is used in conjunction with knowledge of cortisol levels to evaluate adrenal cortical dysfunction. For example, consider the patient with Addison’s disease in which the adrenal cortex is hypoactive, thus producing abnormally low levels of cortisol in the blood. The anterior pituitary gland senses the low serum cortisol levels and, as a result, increases its release of ACTH. This is an attempt to stimulate the adrenal gland to increase its production of cortisol.
Thus, the combination of high ACTH and low cortisol levels indicates adrenocortical hypoactivity. Conversely, if the adrenal gland is overproducing cortisol, as in the presence of an adrenal tumor, the ACTH level will be low as the anterior pituitary gland responds to the elevated cortisol level. Should there be a high level of ACTH due to a pituitary tumor or a nonendocrine ACTH-producing tumor, there will also be an elevated cortisol level, as the adrenal gland responds to stimulation by the ACTH.