Biochemistry of Steroid Hormones

What does 3β-HSD do?

Converts pregnenolone to progesterone.

What is the precursor of all steroid hormones?

Cholesterol.

What does StAR (Steroidogenic acute regulatory protein) do?

Transports cholesterol into mitochondria, where steroid synthesis begins.

What does CYP11A1 (Cholesterol side-chain cleavage enzyme) do?

Converts cholesterol to pregnenolone; this is the first committed, rate-limiting step.

Why is progesterone important in steroid synthesis?

It is a branch point for making cortisol, aldosterone, and sex hormones.

How is cortisol made?

In the adrenal cortex, cholesterol is converted to pregnenolone and progesterone, then through hydroxylation steps to cortisol.

Which enzyme catalyses the final step of cortisol synthesis?

11β-hydroxylase (CYP11B1).

How is aldosterone made?

From cholesterol via pregnenolone and progesterone; aldosterone synthase (CYP11B2) is key to its formation.

How do the key steroid-synthesis enzymes differ?

StAR moves cholesterol into mitochondria; CYP11A1 makes pregnenolone; 3β-HSD makes progesterone; CYP11B1 supports cortisol synthesis; CYP11B2 makes aldosterone.

What controls cortisol release?

Stress activates hypothalamic CRH, which stimulates pituitary ACTH; ACTH stimulates the adrenal cortex to make cortisol.

How does cortisol provide negative feedback?

High cortisol reduces CRH and ACTH release.

How do steroid hormones act on cells?

They cross cell membranes, bind intracellular receptors, and regulate gene transcription; effects are relatively slow but long-lasting.

What are cortisol's main functions?

Stress response, glucose regulation, and immune suppression.

What are aldosterone's main effects?

In kidney distal tubules, it increases sodium reabsorption and potassium excretion, promoting water retention and blood pressure.

What regulates aldosterone?

The renin-angiotensin system and blood potassium levels.

What do cortisol deficiency and excess cause?

Deficiency: Addison's disease. Excess: Cushing's syndrome.

What do aldosterone deficiency and excess cause?

Deficiency: low blood pressure and dehydration. Excess: Conn's syndrome, high blood pressure, and low potassium.

Cortisol vs aldosterone

Cortisol acts mainly in stress and glucose metabolism via glucocorticoid receptors; aldosterone regulates salt and water balance via mineralocorticoid receptors.

What is congenital adrenal hyperplasia (CAH)?

An inherited steroid-synthesis enzyme defect that can lower cortisol (and sometimes aldosterone) and increase androgen production.

What can CAH cause?

Salt loss, low cortisol with increased ACTH and adrenal enlargement, and virilisation in females.