ACTH (adrenocorticotropic hormone) is a hormone produced in the pituitary gland (small gland at the base of the brain) whose role is to stimulate the adrenal glands (glands located on each kidney) to secrete cortisol. Cortisol plays an important role in metabolizing glucose, proteins and lipids (fat), it suppresses the immune response and helps maintain blood pressure. The pituitary gland increases the production of ACTH (and consequently the production of cortisol) in response to various stressors (hypoglycemia, infection, etc.). When cortisol levels are high enough, the production of ACTH stops. Like cortisol, the secretion of ACTH varies throughout the day with maximum blood levels occurring around 8 AM and minimum levels around midnight.
ACTH levels must be interpreted in light of cortisol levels, the time of day and medications taken, particularly corticosteroids. When cortisol levels are high (as a result of an adrenal tumour for example), ACTH should be very low. However, when the adrenal glands are being impacted and cortisol is low, levels of ACTH should be high to compensate. Simultaneously high or simultaneously low levels of ACTH and cortisol are more likely to indicate a pituitary gland condition.