Adrenal Gland Function

Adrenal Gland Structure and Function

• The adrenal gland consists of two main functional parts: the adrenal medulla and adrenal cortex.
• Adrenal medulla: inner part of the adrenal gland, responsible for producing and secreting catecholamines (adrenaline and noradrenaline), involved in the fight or flight response.
• Adrenal cortex: outer part of the adrenal gland, responsible for synthesizing and secreting steroid hormones necessary for physiological functions.

Adrenal Cortex Zones and Functions

• Adrenal cortex is divided into three distinct zones:
  • Zona glomerulosa: outermost layer, produces aldosterone (regulates electrolyte balance, Na and K, and blood pressure).
  • Zona fasciculata: middle layer, produces glucocorticoids (mainly cortisol), involved in regulating metabolism, immune function, and stress response.
  • Zona reticularis: innermost layer, produces adrenal androgens, contributing to secondary sexual characteristics.

Glucocorticoids Functions

• Glucocorticoids increase protein breakdown, glycogenolysis, and gluconeogenesis.
• They also sensitize arterioles to noradrenaline actions, key to controlling blood pressure.
• Glucocorticoids have an effect on water excretion (diuresis).

ACTH Negative Feedback Loop

• The hypothalamus monitors blood cortisol levels and initiates the regulation process.
• When cortisol levels are low, the hypothalamus secretes corticotrophin releasing hormone (CRH), which stimulates the secretion of adrenocorticotropic hormone (ACTH) in the anterior pituitary gland.
• ACTH stimulates the synthesis and secretion of cortisol in the adrenal cortex.
• As blood cortisol levels rise, cortisol exerts negative feedback on the anterior pituitary and hypothalamus.

Cushing's Syndrome

• Cushing's syndrome: persistent and inappropriately raised plasma cortisol levels, often due to exogenous steroids.
• Cushing's disease: glucocorticoid excess secondary to inappropriate ACTH secretion, caused by an ACTH-producing tumor or pituitary tumor.
• Endogenous causes: Spontaneous Cushing's, pituitary adenoma, ectopic ACTH-producing tumor.
• Adrenal adenomas/carcinomas autonomously produce cortisol, leading to Cushing's syndrome.
• Pseudo-Cushing syndrome: alcohol can mimic Cushing's syndrome clinically and biochemically, resolves upon alcohol cessation.

Clinical Features of Cushing's Syndrome

• Obesity: fat centrally distributed (trunk/abdomen/buffalo hump on neck).
• Plethoric complexion and thin skin, easily bruising due to protein catabolic effect.
• Moon face: round and full face.
• Purple striae (stretch marks) on abdomen/breasts/thigh.
• Pigmentation (melanin) if ACTH dependent.

Investigations for Cushing's Syndrome

• Raised urinary free cortisol.
• Elevated cortisol levels throughout the day, loss of circadian rhythm.
• Dexamethasone suppression test: administer 0.5mg Dex every 6hrs for 48hrs, suppresses endogenous cortisol production in healthy individuals.