ACTH

ACTH

Adrenocorticotropic hormone (ACTH), also known as corticotropin, is a polypeptide tropic hormone produced and secreted by the anterior pituitary gland. It is an important component of the hypothalamic-pituitary-adrenal axis and is often produced in response to biological stress. Its principal effects are increased production and release of cortisol by the cortex of the adrenal gland.

Structure

ACTH is a peptide hormone consisting of 39 amino acids. The sequence of ACTH is highly conserved across species, reflecting its critical role in physiology. The first 13 amino acids of ACTH are identical to the sequence of α-melanocyte-stimulating hormone (α-MSH), which explains some of the overlapping functions of these hormones.

Function

ACTH stimulates the adrenal cortex to secrete glucocorticoids, mainly cortisol, and to a lesser extent, mineralocorticoids and androgens. The hormone acts by binding to the melanocortin receptor 2 (MC2R) on the surface of adrenal cortex cells, which activates the enzyme adenylate cyclase, increasing the intracellular concentration of cyclic AMP (cAMP) and triggering the synthesis and release of cortisol.

Regulation

The secretion of ACTH is regulated by the hypothalamus through the release of corticotropin-releasing hormone (CRH). CRH is released in response to stress and low blood-glucocorticoid levels. ACTH secretion follows a circadian rhythm, with peak levels occurring in the early morning and trough levels in the evening.

Clinical Significance

Abnormal levels of ACTH can lead to various disorders:

• Cushing's Disease: Caused by an ACTH-secreting pituitary adenoma, leading to excessive cortisol production.
• Addison's Disease: Characterized by low cortisol levels due to adrenal insufficiency, often resulting in elevated ACTH levels as the body attempts to stimulate the adrenal glands.
• Ectopic ACTH Syndrome: Occurs when ACTH is produced by non-pituitary tumors, such as small cell lung cancer.

Diagnostic Use

ACTH levels can be measured in the blood to help diagnose adrenal and pituitary disorders. The ACTH stimulation test is used to assess adrenal gland function by measuring the adrenal response to synthetic ACTH.

Also see

• Cortisol
• Adrenal gland
• Hypothalamus
• Pituitary gland
• Cushing's syndrome
• Addison's disease