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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# Steroid hormone synthesis (overview)

The adrenals, ovaries, testes and placenta turn cholesterol into steroid hormones; everything starts with pregnenolone.

Updated 2026-11-01

## Key facts

- The rate-limiting step is cholesterol entry into the mitochondrion via StAR, regulated by ACTH, LH or angiotensin II.
- CYP11A1 (side-chain cleavage enzyme) turns cholesterol into pregnenolone, the common precursor.
- Adrenal cortex: glomerulosa → aldosterone; fasciculata → cortisol; reticularis → androgens.
- Gonads: testosterone (testis) and oestradiol (ovary, via aromatase); the placenta makes progesterone.

## In detail

- Cholesterol comes from LDL, from ester stores in lipid droplets or from local synthesis.
- StAR moves it to the inner mitochondrial membrane, where CYP11A1 cuts the side chain: pregnenolone (C21).
- ER and mitochondrial P450s (17α-hydroxylase, 21-hydroxylase, 11β-hydroxylase, aldosterone synthase) and 3β-HSD set each tissue's product.
- Androgens (C19) are converted to oestrogens (C18) by aromatase.
- Vitamin D also comes from cholesterol, starting from 7-dehydrocholesterol in the skin.
- The adrenal enzymes and their deficiencies (congenital adrenal hyperplasia) will get their own chapter.

## Clinical notes

- 21-Hydroxylase deficiency is the most common cause of congenital adrenal hyperplasia.
- Ketoconazole and other P450 inhibitors reduce cortisol production.
- StAR deficiency (lipoid adrenal hyperplasia) blocks all steroid hormones.

## Pathways

- [Adrenal cortisol synthesis](https://durgical.com/en/biochemistry/pathways/adrenal-cortisol-synthesis/)

## Related conditions

- [Cushing syndrome](https://durgical.com/en/conditions/cushing-syndrome/)
- [Adrenal insufficiency](https://durgical.com/en/conditions/adrenal-insufficiency/)
- [Polycystic ovary syndrome](https://durgical.com/en/conditions/polycystic-ovary-syndrome/)

## Related

- [Cholesterol](https://durgical.com/en/biochemistry/molecules/cholesterol/)
- [LDL (low-density lipoprotein)](https://durgical.com/en/biochemistry/topics/ldl/)
- [HDL (high-density lipoprotein)](https://durgical.com/en/biochemistry/topics/hdl/)
- [Cholesterol synthesis (mevalonate pathway)](https://durgical.com/en/biochemistry/pathways/cholesterol-synthesis/)

## Test yourself

Think of the answer, then tap the question to see it.

## What is the rate-limiting step of steroid hormone synthesis?

Cholesterol entry into the mitochondrion via StAR, followed by cleavage by CYP11A1.

## Which molecule is the common precursor of all steroid hormones?

Pregnenolone.

## Which hormone does each adrenal cortex zone make?

Glomerulosa: aldosterone; fasciculata: cortisol; reticularis: androgens.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Metabolism of steroid hormones (R-HSA-196071)](https://reactome.org/PathwayBrowser/#/R-HSA-196071) (Reactome, 2026); [Glucocorticoid biosynthesis (R-HSA-194002)](https://reactome.org/PathwayBrowser/#/R-HSA-194002) (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/topics/steroid-hormone-synthesis/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/topics/steroid-hormone-synthesis/