🧪 Adrenocorticotropic Hormone (ACTH) is a peptide hormone secreted by the anterior pituitary. It stimulates the adrenal cortex to produce glucocorticoids (mainly cortisol) and androgens, playing a key role in stress response, metabolism, and immune regulation.

🧬 Synthesis and Secretion

Production :
- Derived from pro-opiomelanocortin (POMC) in corticotroph cells of the anterior pituitary.
- POMC also gives rise to other peptides (e.g. MSH, β-endorphin).
Regulation :
- Stimulated by CRH (corticotropin-releasing hormone) from the hypothalamus.
- Triggers include stress, hypoglycaemia, circadian rhythm, and inflammation.
- Negative feedback: High cortisol suppresses both CRH and ACTH release.

⚡ Functions of ACTH

Adrenal Cortex Stimulation :
- Acts on zona fasciculata & zona reticularis.
- Promotes secretion of glucocorticoids and adrenal androgens.
Cortisol Effects :
- ↑ Gluconeogenesis → raises blood glucose levels.
- ↑ Protein catabolism & lipolysis.
- Suppresses immunity & inflammation.
- Vital for the “fight-or-flight” stress response.
Androgen Effects :
- Contributes to adrenal androgen production (e.g. DHEA), important for pubertal changes and minor sex hormone supply.

📊 Clinical Significance

ACTH Stimulation (Synacthen) Test :
- Evaluates adrenal reserve by measuring cortisol response to synthetic ACTH.
- Used in diagnosing Addison’s disease and secondary adrenal insufficiency.
Disorders :
- 🌙 Cushing’s Syndrome :
  - Excess cortisol due to pituitary adenoma (Cushing’s disease) or ectopic ACTH (e.g. small cell lung cancer).
- 🌅 Addison’s Disease :
  - Primary adrenal failure → low cortisol despite high ACTH.
  - Secondary causes (pituitary/CRH deficiency) → low ACTH & cortisol.
- 🧬 Congenital Adrenal Hyperplasia (CAH) :
  - Defects in cortisol synthesis → chronic ACTH elevation → adrenal hyperplasia & androgen excess.

⏰ ACTH and Circadian Rhythm

Secretion follows a diurnal pattern:
- Peak: early morning (06:00–09:00).
- Trough: late evening and night.
This rhythm is essential for synchronising cortisol to daily activity and metabolism.

📏 Reference Values

Normal range: 0–47 ng/L (sample at 9am).
Best measured between 09:00–10:00 to match circadian peak.
Short Synacthen Test: A 30-min cortisol <500 nmol/L suggests adrenal insufficiency.

🌟 Summary

ACTH is the central regulator of adrenal glucocorticoid secretion. Disorders of ACTH lead to adrenal excess (Cushing’s) or deficiency (Addison’s). Understanding its physiology and rhythm is critical for interpreting endocrine tests and managing pituitary–adrenal axis disease.

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ACTH (Adrenocorticotropic hormone)