Related Subjects:
|Adrenal Physiology
|Addison's Disease
|Phaeochromocytoma
|Adrenal Adenomas
|Adrenal Cancer
|Cushing Syndrome
|Cushing Disease
|Congenital Adrenal hyperplasia
|Primary hyperaldosteronism (Conn's syndrome)
|ACTH
|McCune Albright syndrome
Overview of Adrenocorticotropic Hormone (ACTH)
Adrenocorticotropic hormone (ACTH) is a peptide hormone produced by the anterior pituitary gland. It plays a critical role in regulating the adrenal cortex's secretion of glucocorticoids, mainly cortisol, which are essential for stress response, metabolism, and immune function.
Synthesis and Secretion
- Production:
- ACTH is synthesized from a precursor protein called pro-opiomelanocortin (POMC).
- POMC is cleaved into several peptides, including ACTH, in the corticotroph cells of the anterior pituitary.
- Regulation:
- ACTH secretion is regulated by corticotropin-releasing hormone (CRH) from the hypothalamus.
- CRH release is stimulated by various factors, including stress, low blood cortisol levels, and circadian rhythms.
- Negative feedback: Elevated cortisol levels inhibit CRH and ACTH secretion.
Function of ACTH
- Stimulation of Adrenal Cortex:
- ACTH binds to specific receptors on the adrenal cortex, particularly in the zona fasciculata and zona reticularis.
- Stimulates the production and release of glucocorticoids (mainly cortisol) and androgens.
- Cortisol Release:
- Cortisol has several vital functions, including regulation of metabolism, immune response, and stress adaptation.
- Increases blood glucose levels by promoting gluconeogenesis and inhibiting glucose uptake by tissues.
- Enhances the breakdown of proteins and lipids.
- Suppresses the immune system and reduces inflammation.
- Androgen Production:
- ACTH stimulates the adrenal cortex to produce androgens, which are precursors for sex hormones.
Clinical Significance
- ACTH Stimulation Test:
- Used to assess adrenal gland function by measuring cortisol response to synthetic ACTH.
- Helps diagnose conditions such as Addison's disease (adrenal insufficiency) and secondary adrenal insufficiency (pituitary dysfunction).
- Disorders of ACTH Secretion:
- Cushing's Syndrome:
- Caused by prolonged exposure to high levels of cortisol.
- Can result from ACTH-secreting pituitary adenomas (Cushing's disease) or ectopic ACTH production (e.g., from lung tumours).
- Addison's Disease:
- Characterized by adrenal insufficiency and low cortisol production.
- Can be primary (due to adrenal damage) or secondary (due to insufficient ACTH production).
- Congenital Adrenal Hyperplasia (CAH):
- A group of genetic disorders affecting cortisol synthesis, leading to increased ACTH levels and adrenal hyperplasia.
ACTH and Circadian Rhythm
- ACTH secretion follows a diurnal pattern, with peak levels in the early morning and lowest levels at night.
- This circadian rhythm is crucial for maintaining appropriate cortisol levels and metabolic functions throughout the day.
Summary
Adrenocorticotropic hormone (ACTH) is a vital hormone produced by the anterior pituitary gland, regulating the adrenal cortex's production of glucocorticoids and androgens. Its secretion is controlled by CRH from the hypothalamus and follows a circadian rhythm. ACTH plays a critical role in stress response, metabolism, and immune function. Understanding its functions and regulation is essential for diagnosing and managing disorders related to adrenal and pituitary function.
