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Subject: Adrenocorticotropic Hormone (ACTH)
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ACTH is a polypeptide hormone produced by the anterior pituitary gland that exists principally as a chain of 39 amino acids, with a molecular mass of approximately 4,500 Da. Its biologic function is to stimulate cortisol secretion by the adrenal cortex. ACTH secretion is in turn controlled by the hypothalamic hormone CRF and by negative feedback from cortisol.
Normal range: <46 pg/mL.
Diagnosis of Addison disease, CAH, Cushing syndrome, adrenal carcinoma, and ectopic ACTH syndrome
Pituitary-dependent Cushing disease
Ectopic ACTH–producing tumors
Secondary adrenocortical insufficiency
Plasma levels of ACTH exhibit a significant diurnal variation. ACTH is normally highest in the early morning (6–8 am) and lowest in the evening (6–11 pm). Cortisol levels are frequently measured at the same time as ACTH.
Because ACTH is released in bursts, its levels in the blood can vary from minute to minute.
ACTH is unstable in blood, and proper handling of specimen is important.
Most commercial RIAs are insensitive and nonspecific, measuring intact ACTH as well as precursors and fragments. Highly sensitive IRMAs measure intact ACTH only.
RIAs are recommended for investigating ectopic ACTH–producing tumors, because some of the tumors secrete ACTH precursors and fragments. IRMAs are more sensitive than RIAs and are useful for investigating disorders of the hypothalamic–pituitary–adrenal system.
Patients taking glucocorticoids may have suppressed levels of ACTH with an apparent high level of cortisol.
Pregnancy, menstruation, and stress increase secretion.